php-doc-en/reference/var/functions/unset.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<refentry xml:id="function.unset" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>unset</refname>
  <refpurpose>Unset a given variable</refpurpose>
 </refnamediv>
 
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>void</type><methodname>unset</methodname>
   <methodparam><type>mixed</type><parameter>var</parameter></methodparam>
   <methodparam choice="opt"><type>mixed</type><parameter>...</parameter></methodparam>
  </methodsynopsis>
  <para>
   <function>unset</function> destroys the specified variables. 
  </para>
  <para>
   The behavior of <function>unset</function> inside of a function
   can vary depending on what type of variable you are attempting to
   destroy.
  </para>
  <para>
   If a globalized variable is <function>unset</function> inside of
   a function, only the local variable is destroyed.  The variable
   in the calling environment will retain the same value as before
   <function>unset</function> was called.
   <informalexample>
    <programlisting role="php">
<![CDATA[
<?php
function destroy_foo() 
{
    global $foo;
    unset($foo);
}

$foo = 'bar';
destroy_foo();
echo $foo;
?>
]]>
    </programlisting>
   </informalexample>
  </para>
  &example.outputs;
  <para>
   <informalexample>
    <screen>
<![CDATA[
bar
]]>
    </screen>
   </informalexample>
  </para>
  <para>
   To <function>unset</function> a global variable
   inside of a function, then use
   the <varname>$GLOBALS</varname> array to do so:
   <informalexample>
    <programlisting role="php">
<![CDATA[
<?php
function foo() 
{
    unset($GLOBALS['bar']);
}

$bar = "something";
foo();
?>
]]>
    </programlisting>
   </informalexample>
  </para>
  <para>
   If a variable that is PASSED BY REFERENCE is
   <function>unset</function> inside of a function, only the local
   variable is destroyed.  The variable in the calling environment
   will retain the same value as before <function>unset</function>
   was called.
   <informalexample>
    <programlisting role="php">
<![CDATA[
<?php
function foo(&$bar) 
{
    unset($bar);
    $bar = "blah";
}

$bar = 'something';
echo "$bar\n";

foo($bar);
echo "$bar\n";
?>
]]>
    </programlisting>
   </informalexample>
  </para>
   &example.outputs;
  <para>
   <informalexample>
    <screen>
<![CDATA[
something
something
]]>
    </screen>
   </informalexample>
  </para>
  <para>
   If a static variable is <function>unset</function> inside of a
   function, <function>unset</function> destroys the variable only in the
   context of the rest of a function. Following calls will restore the
   previous value of a variable.
   <informalexample>
    <programlisting role="php">
<![CDATA[
<?php
function foo()
{
    static $bar;
    $bar++;
    echo "Before unset: $bar, ";
    unset($bar);
    $bar = 23;
    echo "after unset: $bar\n";
}

foo();
foo();
foo();
?>
]]>
    </programlisting>
   </informalexample>
  </para>
   &example.outputs;
  <para>
   <informalexample>
    <screen>
<![CDATA[
Before unset: 1, after unset: 23
Before unset: 2, after unset: 23
Before unset: 3, after unset: 23
]]>
    </screen>
   </informalexample>
  </para>
 </refsect1>
  
 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>var</parameter></term>
     <listitem>
      <para>
       The variable to be unset.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>...</parameter></term>
     <listitem>
      <para>
       Another variable ...
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   &return.void;
  </para>
 </refsect1>

 <refsect1 role="changelog">
  &reftitle.changelog;
  <para>
   <informaltable>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>&Version;</entry>
       <entry>&Description;</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>4.0.1</entry>
       <entry>
        Added support for multiple arguments.
       </entry>
      </row>
     </tbody>
    </tgroup>
   </informaltable>
  </para>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title><function>unset</function> example</title>
    <programlisting role="php">
<![CDATA[
<?php
// destroy a single variable
unset($foo);

// destroy a single element of an array
unset($bar['quux']);

// destroy more than one variable
unset($foo1, $foo2, $foo3);
?>
]]>
    </programlisting>
   </example>
   <example>
    <title>Using <literal>(unset)</literal> casting</title>
    <para>
     <literal>(unset)</literal> casting is often confused with the
     <function>unset</function> function. <literal>(unset)</literal>
     casting serves only as a <literal>NULL</literal>-type cast, for
     completeness. It does not alter the variable it's casting.
    </para>
    <programlisting role="php">
<![CDATA[
<?php
$name = 'Felipe';

var_dump((unset) $name);
var_dump($name);
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
NULL
string(6) "Felipe"
]]>
    </screen>
   </example>
  </para>
 </refsect1>

 <refsect1 role="notes">
  &reftitle.notes;
  &note.language-construct;
  <note>
   <para>
    It is possible to unset even object properties visible in current context.
   </para>
  </note>
  <note>
   <para>
    It is not possible to unset <literal>$this</literal> inside an object
    method since PHP 5.
   </para>
  </note>
  <note>
   <para>
    When using <function>unset</function> on inaccessible object properties,
    the <link linkend="object.unset">__unset()</link>
    overloading method will be called, if declared.
   </para>
  </note>
 </refsect1>
 
 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>isset</function></member>
    <member><function>empty</function></member>
    <member><link linkend="object.unset">__unset()</link></member>
    <member><function>array_splice</function></member>
   </simplelist>
  </para>
 </refsect1>

</refentry>

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