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 gives the current line number of the 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 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 gives the function name.
CLASS gives the class name.