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defined — Checks whether a constant is defined.


bool defined(constant_name);
string constant_name: Name of the constant to check


TRUE if the constant is defined; FALSE otherwise


defined() is used to check whether the constant named in the constant_name argument is defined.

If the constant is defined, the function returns TRUE. If the constant is not defined, the function returns FALSE.


PHP Version: 3+, 4+

See also

To define a constant:


To check whether a class is defined:


To check whether a function is defined:


To check whether a variable is defined:



Example 749. Check whether a constant is defined

$constant_name = 'FOO';

if (defined ($constant_name))
    echo "Constant <i>$constant_name</i> is defined.";
    echo "Constant <i>$constant_name</i> is not defined.";

PHP Functions Essential Reference. Copyright © 2002 by New Riders Publishing (Authors: Zak Greant, Graeme Merrall, Torben Wilson, Brett Michlitsch). This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0 or later (the latest version is presently available at The authors of this book have elected not to choose any options under the OPL. This online book was obtained from and is designed to provide information about the PHP programming language, focusing on PHP version 4.0.4 for the most part. The information is provided on an as-is basis, and no warranty or fitness is implied. All persons and entities shall have neither liability nor responsibility to any person or entity with respect to any loss or damage arising from the information contained in this book.

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