PHP Manual
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defined

(PHP 3, PHP 4 , PHP 5)

defined --  Checks whether a given named constant exists

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.

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