(PHP 3, PHP 4, PHP 5)

defined --  Checks whether a given named constant exists


bool defined ( string name )

Returns TRUE if the named constant given by name has been defined, FALSE otherwise.

例子 1. Checking Constants

/* 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')) {

注: If you want to see if a variable exists, use isset() as defined() only applies to constants. If you want to see if a function exists, use function_exists().

See also define(), constant(), get_defined_constants(), function_exists(), and the section on Constants.

add a note add a note User Contributed Notes
Harald Ponce de Leon
18-May-2006 11:24
Beware that some PHP versions return an integer (1 or 0) instead of a boolean.

Confirmed PHP versions that return an integer are 4.3.2 and 4.3.4.

Relevant bug report:

This make it impossible to use the following, when the PHP version is not known:

if (defined('CONSTANT') === true) {

Relevant commit for PHP 4.3.5 (thanks to Pollita at #php.thinktank):
ndove at cox dot net
28-Jan-2005 11:20
In PHP5, you can actually use defined() to see if an object constant has been defined, like so:


class Generic
WhatAmI = 'Generic';

if (


Thought it may be useful to note.

Craig at chatspike dot net
01-Dec-2003 03:57
This can be useful if you want to protect pages which get included from outsiders eyes, on your mail page (the page viewable by people) put define("X", null); then on all your other pages, you can then do something like:

if (!defined("X")) {
   echo "You Cannot Access This Script Directly, Have a Nice Day.";

And your page is a good as protected :)