php-doc-en/language/types/null.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<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>

 <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 constant &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 &null;</title>

  <para>
   &warn.deprecated.feature-7-2-0;
  </para>

  <para>
   Casting a variable to <type>null</type> using <literal>(unset) $var</literal>
   will <emphasis>not</emphasis> remove the variable or unset its value.
   It will only return a &null; value.
  </para>

 </sect2>

</sect1>
 
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