Magic constants

PHP provides a large number of predefined constants to any script which it runs. Many of these constants, however, are created by various extensions, and will only be present when those extensions are available, either via dynamic loading or because they have been compiled in.

There are seven magical constants that change depending on where they are used. For example, the value of __LINE__ depends on the line that it's used on in your script. These special constants are case-insensitive and are as follows:

A few "magical" PHP constants Name     Description

__LINE__     The current line number of the file.
__FILE__     The full path and filename of the file. If used inside an include, the name of the included file is returned.
__DIR__     The directory of the file. If used inside an include, the directory of the included file is returned. This is equivalent to dirname(__FILE__).
__FUNCTION__     The function name.  As of PHP 5 this constant returns the function name as it was declared (case-sensitive).
__CLASS__     The class name.
__METHOD__     The class method name.
__NAMESPACE__     The name of the current namespace (case-sensitive). This constant is defined in compile-time