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<refentry xml:id="function.isset" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>isset</refname>
  <refpurpose>Determine if a variable is declared and is different than &null;</refpurpose>
 </refnamediv>
 
 <refsect1 role="description">
 &reftitle.description;
  <methodsynopsis>
   <type>bool</type><methodname>isset</methodname>
   <methodparam><type>mixed</type><parameter>var</parameter></methodparam>
   <methodparam rep="repeat"><type>mixed</type><parameter>vars</parameter></methodparam>
  </methodsynopsis>
  <para>
   Determine if a variable is considered set,
   this means if a variable is declared and is different than &null;.
  </para>
  <para>
   If a variable has been unset with the <function>unset</function>
   function, it is no longer considered to be set.
  </para>
  <para>
   <function>isset</function> will return &false; when checking a
   variable that has been assigned to &null;.
   Also note that a null character (<literal>"\0"</literal>) is not
   equivalent to the PHP &null; constant.
  </para>
  <para>
   If multiple parameters are supplied then <function>isset</function>
   will return &true; only if all of the parameters are considered set.
   Evaluation goes from left to right and stops as soon as an unset
   variable is encountered.
  </para>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>var</parameter></term>
     <listitem>
      <para>
       The variable to be checked.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>vars</parameter></term>
     <listitem>
      <para>
       Further variables.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   Returns &true; if <parameter>var</parameter> exists and has
   any value other than &null;. &false; otherwise.
  </para>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title><function>isset</function> Examples</title>
    <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>
   </example>
  </para>
  <para>
   This also work for elements in arrays:
   <informalexample>
    <programlisting role="php">
<![CDATA[
<?php

$a = array ('test' => 1, 'hello' => NULL, 'pie' => array('a' => 'apple'));

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

// Checking deeper array values
var_dump(isset($a['pie']['a']));        // TRUE
var_dump(isset($a['pie']['b']));        // FALSE
var_dump(isset($a['cake']['a']['b']));  // FALSE

?>
]]>
    </programlisting>
   </informalexample>
  </para>
  <example>
   <title><function>isset</function> on String Offsets</title>
   <programlisting role="php">
<![CDATA[
<?php
$expected_array_got_string = 'somestring';
var_dump(isset($expected_array_got_string['some_key']));
var_dump(isset($expected_array_got_string[0]));
var_dump(isset($expected_array_got_string['0']));
var_dump(isset($expected_array_got_string[0.5]));
var_dump(isset($expected_array_got_string['0.5']));
var_dump(isset($expected_array_got_string['0 Mostel']));
?>
]]>
   </programlisting>
   &example.outputs;
   <screen>
<![CDATA[
bool(false)
bool(true)
bool(true)
bool(true)
bool(false)
bool(false)
]]>
   </screen>
  </example>
 </refsect1>

 <refsect1 role="notes">
  &reftitle.notes;
  <warning>
   <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>
  </warning>
  &note.language-construct;
  <note>
   <para>
    When using <function>isset</function> on inaccessible object properties,
    the <link linkend="object.isset">__isset()</link>
    overloading method will be called, if declared.
   </para>
  </note>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>empty</function></member>
    <member><link linkend="object.isset">__isset()</link></member>
    <member><function>unset</function></member>
    <member><function>defined</function></member>
    <member><link linkend="types.comparisons">the type comparison tables</link></member>
    <member><function>array_key_exists</function></member>
    <member><function>is_null</function></member>
    <member>the error control <link linkend="language.operators.errorcontrol">@</link> operator</member>
   </simplelist>
  </para>
 </refsect1>

</refentry>

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