php-doc-en/reference/funchand/functions/function-exists.xml

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<refentry xml:id="function.function-exists" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>function_exists</refname>
  <refpurpose>Return &true; if the given function has been defined</refpurpose>
 </refnamediv>
 
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>bool</type><methodname>function_exists</methodname>
   <methodparam><type>string</type><parameter>function_name</parameter></methodparam>
  </methodsynopsis>
  <para>
   Checks the list of defined functions, both built-in (internal) and
   user-defined, for <parameter>function_name</parameter>. 
  </para>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>function_name</parameter></term>
     <listitem>
      <para>
       The function name, as a string.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   Returns &true; if <parameter>function_name</parameter> exists and is a
   function, &false; otherwise.
  </para>
  <note>
   <para>
    This function will return &false; for constructs, such as 
    <function>include_once</function> and <function>echo</function>.
   </para>
  </note>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title><function>function_exists</function> example</title>
    <programlisting role="php">
<![CDATA[
<?php
if (function_exists('imap_open')) {
    echo "IMAP functions are available.<br />\n";
} else {
    echo "IMAP functions are not available.<br />\n";
}
?>
]]>
    </programlisting>
   </example>
  </para>
 </refsect1>

 <refsect1 role="notes">
  &reftitle.notes;
  <note>
   <para>
    A function name may exist even if the function itself is unusable due to
    configuration or compiling options (with the <link
    linkend="ref.image">image</link> functions being an example).
   </para>
  </note>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>method_exists</function></member>
    <member><function>is_callable</function></member>
    <member><function>get_defined_functions</function></member>
   </simplelist>
  </para>
 </refsect1>

</refentry>

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