PHP Functions Essential Reference

defined

bool defined(string constant_name) 

Checks whether a constant is defined.

Returns:

TRUE if the constant is defined; FALSE otherwise

Description:

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.

Availability:

UNIX/Linux, Windows

Version:

3+, 4+

See also:

To define a constant:

define() 

To check whether a class is defined:

class_exists() 

To check whether a function is defined:

function_exists() 

To check whether a variable is defined:

isset() 

Example:

Check whether a constant is defined

$constant_name = 'FOO'; if (defined ($constant_name)) echo "Constant <i>$constant_name</i> ...