defined

Description

bool defined ( string name )

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

例子 1. Checking Constants
<?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; } ?>

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 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:



http://bugs.php.net/bug.php?id=27443



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):



http://cvs.php.net/viewcvs.cgi/Zend/zend_builtin_functions.c?

r1=1.124.2.13&r2=1.124.2.14

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:



<?php



class Generic

{

    const WhatAmI = 'Generic';

}



if (defined('Generic::WhatAmI'))

{

    echo Generic::WhatAmI;

}



?>



Thought it may be useful to note.



-Nick

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.";

    exit();

}



And your page is a good as protected :)

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