PHP Language Constants
PHP provides a large number of predefined constants to any script which it runs. These constants change depending on where they are used. These special constants are case-insensitive.
__LINE__
LINE gives the current line number of the file.
__FILE__
FILE gives the full path and file name of the file with symlinks resolved. If used inside an include, the name of the included file is returned.
__DIR__
DIR gives the directory of the file. If used inside an include, the directory of the included file is returned. This is equivalent to dirname(__FILE__). This directory name does not have a trailing slash unless it is the root directory.
__FUNCTION__
FUNCTION gives the function name.
__CLASS__
CLASS gives the class name.