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<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision: 288721 $ -->
<sect1 xml:id="language.types.null">
<title>NULL</title>
<para>
The special &null; value represents a variable with no value. &null; is the
only possible value of type <type>NULL</type>.
</para>
<note>
<simpara>
The <type>null</type> type was introduced in PHP 4.
</simpara>
</note>
<para>
A variable is considered to be <type>null</type> if:
</para>
<itemizedlist>
<listitem>
<para>
it has been assigned the constant &null;.
</para>
</listitem>
<listitem>
<para>
it has not been set to any value yet.
</para>
</listitem>
<listitem>
<para>
it has been <function>unset</function>.
</para>
</listitem>
</itemizedlist>
<sect2 xml:id="language.types.null.syntax">
<title>Syntax</title>
<para>
There is only one value of type <type>null</type>, and that is the
case-insensitive keyword &null;.
</para>
<informalexample>
<programlisting role="php">
<![CDATA[
<?php
$var = NULL;
?>
]]>
</programlisting>
</informalexample>
<para>
See also the functions <function>is_null</function> and
<function>unset</function>.
</para>
</sect2>
<sect2 xml:id="language.types.null.casting">
<title>Casting to <literal>NULL</literal></title>
<para>
Casting a variable to <type>null</type> will remove the variable
and unset its value.
</para>
</sect2>
</sect1>
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