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641 lines
18 KiB
XML
641 lines
18 KiB
XML
<?xml version="1.0" encoding="iso-8859-1"?>
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<!-- $Revision: 1.31 $ -->
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<chapter id="language.operators">
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<title>Operators</title>
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<simpara>
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</simpara>
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<sect1 id="language.operators.arithmetic">
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<title>Arithmetic Operators</title>
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<simpara>
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Remember basic arithmetic from school? These work just
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like those.
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</simpara>
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<table>
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<title>Arithmetic Operators</title>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Example</entry>
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<entry>Name</entry>
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<entry>Result</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>$a + $b</entry>
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<entry>Addition</entry>
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<entry>Sum of $a and $b.</entry>
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</row>
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<row>
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<entry>$a - $b</entry>
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<entry>Subtraction</entry>
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<entry>Difference of $a and $b.</entry>
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</row>
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<row>
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<entry>$a * $b</entry>
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<entry>Multiplication</entry>
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<entry>Product of $a and $b.</entry>
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</row>
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<row>
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<entry>$a / $b</entry>
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<entry>Division</entry>
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<entry>Quotient of $a and $b.</entry>
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</row>
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<row>
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<entry>$a % $b</entry>
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<entry>Modulus</entry>
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<entry>Remainder of $a divided by $b.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<simpara>
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The division operator ("/") returns an integer value (the result
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of an integer division) if the two operands are integers (or
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strings that get converted to integers) and the quotient is an
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integer. If either operand is a floating-point value, or the
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operation results in a non-integer value, a floating-point value
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is returned.
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</simpara>
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</sect1>
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<sect1 id="language.operators.assignment">
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<title>Assignment Operators</title>
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<simpara>
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The basic assignment operator is "=". Your first inclination might
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be to think of this as "equal to". Don't. It really means that the
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the left operand gets set to the value of the expression on the
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rights (that is, "gets set to").
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</simpara>
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<para>
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The value of an assignment expression is the value assigned. That
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is, the value of "$a = 3" is 3. This allows you to do some tricky
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things:
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<informalexample>
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<programlisting role="php">
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<![CDATA[
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$a = ($b = 4) + 5; // $a is equal to 9 now, and $b has been set to 4.
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]]>
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</programlisting>
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</informalexample>
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</para>
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<para>
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In addition to the basic assignment operator, there are "combined
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operators" for all of the binary arithmetic and string operators
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that allow you to use a value in an expression and then set its
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value to the result of that expression. For example:
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<informalexample>
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<programlisting role="php">
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<![CDATA[
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$a = 3;
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$a += 5; // sets $a to 8, as if we had said: $a = $a + 5;
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$b = "Hello ";
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$b .= "There!"; // sets $b to "Hello There!", just like $b = $b . "There!";
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]]>
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</programlisting>
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</informalexample>
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</para>
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<para>
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Note that the assignment copies the original variable to the new
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one (assignment by value), so changes to one will not affect the
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other. This may also have relevance if you need to copy something
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like a large array inside a tight loop. PHP 4 supports assignment
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by reference, using the <computeroutput>$var =
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&$othervar;</computeroutput> syntax, but this is not possible
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in PHP 3. 'Assignment by reference' means that both variables end
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up pointing at the same data, and nothing is copied anywhere.
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To learn more about references, please read <link
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linkend="language.references">References explained</link>.
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</para>
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</sect1>
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<sect1 id="language.operators.bitwise">
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<title>Bitwise Operators</title>
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<simpara>
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Bitwise operators allow you to turn specific bits within an
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integer on or off.
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</simpara>
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<table>
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<title>Bitwise Operators</title>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Example</entry>
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<entry>Name</entry>
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<entry>Result</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>$a & $b</entry>
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<entry>And</entry>
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<entry>Bits that are set in both $a and $b are set.</entry>
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</row>
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<row>
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<entry>$a | $b</entry>
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<entry>Or</entry>
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<entry>Bits that are set in either $a or $b are set.</entry>
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</row>
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<row>
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<entry>$a ^ $b</entry>
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<entry>Xor</entry>
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<entry>
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Bits that are set in $a or $b but not both are set.
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</entry>
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</row>
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<row>
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<entry>~ $a</entry>
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<entry>Not</entry>
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<entry>
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Bits that are set in $a are not set, and vice versa.
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</entry>
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</row>
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<row>
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<entry>$a << $b</entry>
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<entry>Shift left</entry>
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<entry>
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Shift the bits of $a $b steps to the left (each step means
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"multiply by two")
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</entry>
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</row>
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<row>
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<entry>$a >> $b</entry>
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<entry>Shift right</entry>
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<entry>
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Shift the bits of $a $b steps to the right (each step means
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"divide by two")
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</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</sect1>
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<sect1 id="language.operators.comparison">
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<title>Comparison Operators</title>
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<simpara>
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Comparison operators, as their name implies, allow you to compare
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two values.
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</simpara>
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<table>
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<title>Comparison Operators</title>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Example</entry>
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<entry>Name</entry>
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<entry>Result</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>$a == $b</entry>
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<entry>Equal</entry>
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<entry>&true; if $a is equal to $b.</entry>
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</row>
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<row>
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<entry>$a === $b</entry>
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<entry>Identical</entry>
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<entry>
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&true; if $a is equal to $b, and they are of the same
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type. (PHP 4 only)
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</entry>
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</row>
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<row>
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<entry>$a != $b</entry>
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<entry>Not equal</entry>
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<entry>&true; if $a is not equal to $b.</entry>
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</row>
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<row>
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<entry>$a <> $b</entry>
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<entry>Not equal</entry>
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<entry>&true; if $a is not equal to $b.</entry>
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</row>
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<row>
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<entry>$a !== $b</entry>
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<entry>Not identical</entry>
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<entry>
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&true; if $a is not equal to $b, or they are not of the same
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type. (PHP 4 only)
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</entry>
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</row>
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<row>
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<entry>$a < $b</entry>
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<entry>Less than</entry>
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<entry>&true; if $a is strictly less than $b.</entry>
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</row>
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<row>
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<entry>$a > $b</entry>
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<entry>Greater than</entry>
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<entry>&true; if $a is strictly greater than $b.</entry>
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</row>
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<row>
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<entry>$a <= $b</entry>
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<entry>Less than or equal to </entry>
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<entry>&true; if $a is less than or equal to $b.</entry>
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</row>
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<row>
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<entry>$a >= $b</entry>
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<entry>Greater than or equal to </entry>
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<entry>&true; if $a is greater than or equal to $b.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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Another conditional operator is the "?:" (or ternary) operator,
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which operates as in C and many other languages.
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<informalexample>
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<programlisting>
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<![CDATA[
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(expr1) ? (expr2) : (expr3);
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]]>
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</programlisting>
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</informalexample>
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This expression evaluates to <replaceable>expr2</replaceable> if
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<replaceable>expr1</replaceable> evaluates to &true;, and
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<replaceable>expr3</replaceable> if
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<replaceable>expr1</replaceable> evaluates to &false;.
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</para>
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</sect1>
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<sect1 id="language.operators.errorcontrol">
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<title>Error Control Operators</title>
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<simpara>
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PHP supports one error control operator: the at sign (@). When
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prepended to an expression in PHP, any error messages that might
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be generated by that expression will be ignored.
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</simpara>
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<simpara>
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If the <link linkend="ini.track-errors">track_errors</link>
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feature is enabled, any error message generated by the expression
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will be saved in the global variable $php_errormsg. This variable
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will be overwritten on each error, so check early if you want to
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use it.
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</simpara>
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<para>
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<informalexample>
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<programlisting role="php">
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<![CDATA[
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<?php
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/* Intentional file error */
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$my_file = @file ('non_existent_file') or
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die ("Failed opening file: error was '$php_errormsg'");
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// this works for any expression, not just functions:
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$value = @$cache[$key];
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// will not issue a notice if the index $key doesn't exist.
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?>
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]]>
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</programlisting>
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</informalexample>
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</para>
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<note>
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<simpara>
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The @-operator works only on expressions. A simple rule of thumb
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is: if you can take the value of something, you can prepend the @
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operator to it. For instance, you can prepend it to variables,
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function and <function>include</function> calls, constants, and
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so forth. You cannot prepend it to function or class definitions,
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or conditional structures such as <literal>if</literal> and
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<literal>foreach</literal>, and so forth.
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</simpara>
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</note>
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<simpara>
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See also <function>error_reporting</function>.
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</simpara>
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<warning>
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<para>
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Currently the "@" error-control operator prefix will even disable
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error reporting for critical errors that will terminate script
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execution. Among other things, this means that if you use "@" to
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suppress errors from a certain function and either it isn't
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available or has been mistyped, the script will die right there
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with no indication as to why.
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</para>
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</warning>
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</sect1>
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<sect1 id="language.operators.execution">
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<title>Execution Operators</title>
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<para>
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PHP supports one execution operator: backticks (``). Note that
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these are not single-quotes! PHP will attempt to execute the
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contents of the backticks as a shell command; the output will be
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returned (i.e., it won't simply be dumped to output; it can be
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assigned to a variable).
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<informalexample>
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<programlisting role="php">
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<![CDATA[
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$output = `ls -al`;
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echo "<pre>$output</pre>";
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]]>
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</programlisting>
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</informalexample>
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</para>
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<note>
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<para>
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The backtick operator is disabled when <link
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linkend="ini.safe-mode">safe mode</link> is enabled
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or <function>shell_exec</function> is disabled.
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</para>
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</note>
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<para>
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See also <function>escapeshellcmd</function>, <function>exec</function>,
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<function>passthru</function>, <function>popen</function>,
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<function>shell_exec</function>, and <function>system</function>.
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</para>
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</sect1>
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<sect1 id="language.operators.increment">
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<title>Incrementing/Decrementing Operators</title>
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<para>
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PHP supports C-style pre- and post-increment and decrement
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operators.
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</para>
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<table>
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<title>Increment/decrement Operators</title>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Example</entry>
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<entry>Name</entry>
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<entry>Effect</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>++$a</entry>
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<entry>Pre-increment</entry>
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<entry>Increments $a by one, then returns $a.</entry>
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</row>
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<row>
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<entry>$a++</entry>
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<entry>Post-increment</entry>
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<entry>Returns $a, then increments $a by one.</entry>
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</row>
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<row>
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<entry>--$a</entry>
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<entry>Pre-decrement</entry>
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<entry>Decrements $a by one, then returns $a.</entry>
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</row>
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<row>
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<entry>$a--</entry>
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<entry>Post-decrement</entry>
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<entry>Returns $a, then decrements $a by one.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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Here's a simple example script:
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<informalexample>
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<programlisting role="php">
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<![CDATA[
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<?php
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echo "<h3>Postincrement</h3>";
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$a = 5;
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echo "Should be 5: " . $a++ . "<br>\n";
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echo "Should be 6: " . $a . "<br>\n";
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echo "<h3>Preincrement</h3>";
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$a = 5;
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echo "Should be 6: " . ++$a . "<br>\n";
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echo "Should be 6: " . $a . "<br>\n";
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echo "<h3>Postdecrement</h3>";
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$a = 5;
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echo "Should be 5: " . $a-- . "<br>\n";
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echo "Should be 4: " . $a . "<br>\n";
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echo "<h3>Predecrement</h3>";
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$a = 5;
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echo "Should be 4: " . --$a . "<br>\n";
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echo "Should be 4: " . $a . "<br>\n";
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?>
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]]>
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</programlisting>
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</informalexample>
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</para>
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</sect1>
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<sect1 id="language.operators.logical">
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<title>Logical Operators</title>
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<table>
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<title>Logical Operators</title>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Example</entry>
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<entry>Name</entry>
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<entry>Result</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>$a and $b</entry>
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<entry>And</entry>
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<entry>&true; if both $a and $b are &true;.</entry>
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</row>
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<row>
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<entry>$a or $b</entry>
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<entry>Or</entry>
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<entry>&true; if either $a or $b is &true;.</entry>
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</row>
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<row>
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<entry>$a xor $b</entry>
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<entry>Xor</entry>
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<entry>&true; if either $a or $b is &true;, but not both.</entry>
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</row>
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<row>
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<entry>! $a</entry>
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<entry>Not</entry>
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<entry>&true; if $a is not &true;.</entry>
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</row>
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<row>
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<entry>$a && $b</entry>
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<entry>And</entry>
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<entry>&true; if both $a and $b are &true;.</entry>
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</row>
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<row>
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<entry>$a || $b</entry>
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<entry>Or</entry>
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<entry>&true; if either $a or $b is &true;.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<simpara>
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The reason for the two different variations of "and" and "or"
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operators is that they operate at different precedences. (See
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<link linkend="language.operators.precedence">Operator
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Precedence</link>.)
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</simpara>
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</sect1>
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<sect1 id="language.operators.precedence">
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<title>Operator Precedence</title>
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<para>
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The precedence of an operator specifies how "tightly" it binds two
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expressions together. For example, in the expression <literal>1 +
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5 * 3</literal>, the answer is <literal>16</literal> and not
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<literal>18</literal> because the multiplication ("*") operator
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has a higher precedence than the addition ("+") operator.
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Parentheses may be used to force precedence, if necessary. For
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instance: <literal>(1 + 5) * 3</literal> evaluates to
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<literal>18</literal>.
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</para>
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<para>
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The following table lists the precedence of operators with the
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lowest-precedence operators listed first.
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<table>
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<title>Operator Precedence</title>
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<tgroup cols="2">
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<thead>
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<row>
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<entry>Associativity</entry>
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<entry>Operators</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>left</entry>
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<entry>,</entry>
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</row>
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<row>
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<entry>left</entry>
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<entry>or</entry>
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</row>
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<row>
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<entry>left</entry>
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<entry>xor</entry>
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</row>
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<row>
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<entry>left</entry>
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<entry>and</entry>
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</row>
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<row>
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<entry>right</entry>
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<entry>print</entry>
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</row>
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|
<row>
|
|
<entry>left</entry>
|
|
<entry>
|
|
= += -= *= /= .= %= &= |= ^= ~= <<= >>=
|
|
</entry>
|
|
</row>
|
|
<row>
|
|
<entry>left</entry>
|
|
<entry>? :</entry>
|
|
</row>
|
|
<row>
|
|
<entry>left</entry>
|
|
<entry>||</entry>
|
|
</row>
|
|
<row>
|
|
<entry>left</entry>
|
|
<entry>&&</entry>
|
|
</row>
|
|
<row>
|
|
<entry>left</entry>
|
|
<entry>|</entry>
|
|
</row>
|
|
<row>
|
|
<entry>left</entry>
|
|
<entry>^</entry>
|
|
</row>
|
|
<row>
|
|
<entry>left</entry>
|
|
<entry>&</entry>
|
|
</row>
|
|
<row>
|
|
<entry>non-associative</entry>
|
|
<entry>== != === !==</entry>
|
|
</row>
|
|
<row>
|
|
<entry>non-associative</entry>
|
|
<entry>< <= > >=</entry>
|
|
</row>
|
|
<row>
|
|
<entry>left</entry>
|
|
<entry><< >></entry>
|
|
</row>
|
|
<row>
|
|
<entry>left</entry>
|
|
<entry>+ - .</entry>
|
|
</row>
|
|
<row>
|
|
<entry>left</entry>
|
|
<entry>* / %</entry>
|
|
</row>
|
|
<row>
|
|
<entry>right</entry>
|
|
<entry>! ~ ++ -- (int) (double) (string) (array) (object) @</entry>
|
|
</row>
|
|
<row>
|
|
<entry>right</entry>
|
|
<entry>[</entry>
|
|
</row>
|
|
<row>
|
|
<entry>non-associative</entry>
|
|
<entry>new</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1 id="language.operators.string">
|
|
<title>String Operators</title>
|
|
<simpara>
|
|
There are two string operators. The first is the concatenation
|
|
operator ('.'), which returns the concatenation of its right and
|
|
left arguments. The second is the concatenating assignment
|
|
operator ('.='), which appends the argument on the right side to
|
|
the argument on the left side. Please read <link
|
|
linkend="language.operators.assignment">Assignment
|
|
Operators</link> for more information.
|
|
</simpara>
|
|
|
|
<para>
|
|
<informalexample>
|
|
<programlisting role="php">
|
|
<![CDATA[
|
|
$a = "Hello ";
|
|
$b = $a . "World!"; // now $b contains "Hello World!"
|
|
|
|
$a = "Hello ";
|
|
$a .= "World!"; // now $a contains "Hello World!"
|
|
]]>
|
|
</programlisting>
|
|
</informalexample>
|
|
</para>
|
|
</sect1>
|
|
|
|
</chapter>
|
|
|
|
<!-- Keep this comment at the end of the file
|
|
Local variables:
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mode: sgml
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sgml-omittag:t
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sgml-shorttag:t
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sgml-minimize-attributes:nil
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sgml-always-quote-attributes:t
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sgml-indent-step:1
|
|
sgml-indent-data:t
|
|
indent-tabs-mode:nil
|
|
sgml-parent-document:nil
|
|
sgml-default-dtd-file:"../../manual.ced"
|
|
sgml-exposed-tags:nil
|
|
sgml-local-catalogs:nil
|
|
sgml-local-ecat-files:nil
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End:
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vim600: syn=xml fen fdm=syntax fdl=2 si
|
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vim: et tw=78 syn=sgml
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|
vi: ts=1 sw=1
|
|
-->
|