Constants in PHP

User Defined Constants

define('LABEL', 'Actual Value')

Magic Constants

Const	The magic it has
__LINE__	The current line number of the file.
__FILE__	The full path and filename of the file with symlinks resolved
__DIR__	The directory of the file. equivalent to dirname(__FILE__).
__FUNCTION__	The function name.
__CLASS__	The class name. The class name includes the namespace it was declared in (e.g. Foo\Bar).
__TRAIT__	The trait name. The trait name includes the namespace it was declared in (e.g. Foo\Bar).
__METHOD__	The class method name.
__NAMESPACE__	The name of the current namespace.

Note : directory name __DIR__ does not have a trailing slash unless it is the root directory.