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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.

示範 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; } ?>`

> 備注: 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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