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<?xml version="1.0" encoding="iso-8859-1"?>
<!-- $Revision: 1.14 $ -->
<!-- splitted from ./en/functions/var.xml, last change in rev 1.2 -->
<refentry id="function.isset">
<refnamediv>
<refname>isset</refname>
<refpurpose>Determine whether a variable is set</refpurpose>
</refnamediv>
<refsect1>
<title>Description</title>
<methodsynopsis>
<type>bool</type><methodname>isset</methodname>
<methodparam><type>mixed</type><parameter>var</parameter></methodparam>
<methodparam choice="opt"><type>mixed</type><parameter>var</parameter></methodparam>
<methodparam choice="opt"><parameter>...</parameter></methodparam>
</methodsynopsis>
<simpara>
Returns &true; if <parameter>var</parameter> exists; &false; otherwise.
</simpara>
<para>
If a variable has been unset with <function>unset</function>, it will no
longer be set. <function>isset</function> will
return &false; if testing a variable that has been set to &null;. Also
note that a &null; byte (<literal>"\0"</literal>) is not equivalent to
the PHP &null; constant.
</para>
<note>
<title>Warning</title>
<para>
<function>isset</function> only works with variables as passing anything
else will result in a parse error. For checking if
<link linkend="language.constants">constants</link> are set use the
<function>defined</function> function.
</para>
</note>
<para>
<informalexample>
<programlisting role="php">
<![CDATA[
<?php
$var = '';
// This will evaluate to TRUE so the text will be printed.
if (isset($var)) {
echo "This var is set so I will print.";
}
// In the next examples we'll use var_dump to output
// the return value of isset().
$a = "test";
$b = "anothertest";
var_dump(isset($a)); // TRUE
var_dump(isset($a, $b)); // TRUE
unset ($a);
var_dump(isset($a)); // FALSE
var_dump(isset($a, $b)); // FALSE
$foo = NULL;
var_dump(isset($foo)); // FALSE
?>
]]>
</programlisting>
</informalexample>
</para>
<para>
This also work for elements in arrays:
<informalexample>
<programlisting role="php">
<![CDATA[
<?php
$a = array ('test' => 1, 'hello' => NULL);
var_dump(isset($a['test'])); // TRUE
var_dump(isset($a['foo'])); // FALSE
var_dump(isset($a['hello'])); // FALSE
// The key 'hello' equals NULL so is considered unset
// If you want to check for NULL key values then try:
var_dump(array_key_exists('hello', $a)); // TRUE
?>
]]>
</programlisting>
</informalexample>
</para>
¬e.language-construct;
<simpara>
See also <function>empty</function>,
<function>unset</function>,
<function>defined</function>,
<link linkend="types.comparisons">the type comparison tables</link>,
<function>array_key_exists</function>,
and the error control
<link linkend="language.operators.errorcontrol">@</link> operator.
</simpara>
</refsect1>
</refentry>
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