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<refentry xml:id="function.defined" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>defined</refname>
  <refpurpose>Checks whether a given named constant exists</refpurpose>
 </refnamediv>
 
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>bool</type><methodname>defined</methodname>
   <methodparam><type>string</type><parameter>name</parameter></methodparam>
  </methodsynopsis>
  <para>
   Checks whether the given constant exists and is defined.
  </para>
  <note>
   <para>
    If you want to see if a variable exists, use <function>isset</function>
    as <function>defined</function> only applies to <link
    linkend="language.constants">constants</link>. If you want to see if a
    function exists, use <function>function_exists</function>.
   </para>
  </note>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>name</parameter></term>
     <listitem>
      <para>
       The constant name.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   Returns &true; if the named constant given by <parameter>name</parameter>
   has been defined, &false; otherwise.
  </para>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title>Checking Constants</title>
    <programlisting role="php">
<![CDATA[
<?php
/* Note the use of quotes, this is important.  This example is checking
 * if the string 'CONSTANT' is the name of a constant named CONSTANT */
if (defined('CONSTANT')) {
    echo CONSTANT;
}
?>
]]>
    </programlisting>
   </example>
  </para>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>define</function></member>
    <member><function>constant</function></member>
    <member><function>get_defined_constants</function></member>
    <member><function>function_exists</function></member>
    <member>The section on <link linkend="language.constants">Constants</link></member>
   </simplelist>
  </para>
 </refsect1>

</refentry>

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