php-doc-en/reference/misc/functions/defined.xml

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  <refentry id="function.defined">
   <refnamediv>
    <refname>defined</refname>
    <refpurpose>
     Checks whether a given named constant exists
    </refpurpose>
   </refnamediv>
   <refsect1>
    <title>Description</title>
     <methodsynopsis>
      <type>bool</type><methodname>defined</methodname>
      <methodparam><type>string</type><parameter>name</parameter></methodparam>
     </methodsynopsis>
    <para>
     Returns &true; if the named constant given by
     <parameter>name</parameter> has been defined, 
     &false; otherwise.
    </para>
    <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>
    <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>
    <para>
     See also <function>define</function>,
     <function>constant</function>,
     <function>get_defined_constants</function>, 
     <function>function_exists</function>, and the section on
     <link linkend="language.constants">Constants</link>.
    </para>
   </refsect1>
  </refentry>

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