php-doc-en/language/oop5/typehinting.xml

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 <sect1 id="language.oop5.typehinting">
  <title>Type Hinting</title>
  <para>
   PHP 5 introduces Type Hinting. Functions are now able to force parameters
   to be objects by specifying the name of the class in the function prototype.
  </para>
  
  <example>
   <title>Type Hinting example</title>
    <programlisting role="php">
<![CDATA[
<?php
// An example class
class MyClass
{
    /**
     * A test function
     *
     * First parameter must be an object of type OtherClass
     */
    public function test(OtherClass $otherclass) {
        echo $otherclass->var;
    }
}

// Another example class
class OtherClass {
    public $var = 'Hello World';
}
?>
]]>
   </programlisting>
   <para>
    Failing to satisfy the type hint results in a fatal error.
   </para>
    <programlisting role="php">
<![CDATA[
<?php
// An instance of each class
$myclass = new MyClass;
$otherclass = new OtherClass;

// Fatal Error: Argument 1 must be an object of class OtherClass
$myclass->test('hello');

// Fatal Error: Argument 1 must be an instance of OtherClass
$foo = new stdClass;
$myclass->test($foo);

// Fatal Error: Argument 1 must not be null
$myclass->test(null);

// Works: Prints Hello World
$myclass->test($otherclass);
?>
]]>
   </programlisting>
   <para>
    Type hinting also works with functions:
   </para>
    <programlisting role="php">
<![CDATA[
<?php
// An example class
class MyClass {
    public $var = 'Hello World';
}

/**
 * A test function
 *
 * First parameter must be an object of type MyClass
 */
function MyFunction (MyClass $foo) {
    echo $foo->var;
}

// Works
$myclass = new MyClass;
MyFunction($myclass);
?>
]]>
   </programlisting>
  </example>
  <para>
   Type Hints can only be of the <type>object</type> type. Traditional
   type hinting with <type>int</type> and <type>string</type> are not
   supported.
  </para>
 </sect1>
 
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