php man page for defined

DEFINED(3)								 1								DEFINED(3)

defined - Checks whether a given named constant exists

SYNOPSIS
       bool defined (string  $name)

DESCRIPTION
	Checks whether the given constant exists and is defined.

       Note

	       If  you	want  to  see if a variable exists, use isset(3) as defined(3) only applies to constants. If you want to see if a function
	      exists, use function_exists(3).

PARAMETERS
	      o $name
		- The constant name.

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

EXAMPLES
       Example #1

	      Checking Constants

	      <?php
	      /* Note the use of quotes, this is important.  This example is checking
	       * if the string 'TEST' is the name of a constant named TEST */
	      if (defined('TEST')) {
		  echo TEST;
	      }
	      ?>

SEE ALSO
       define(3), constant(3), get_defined_constants(3), function_exists(3), The section on Constants.

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