Magical Constants
To allow to determine the line number, the full path and filename of the file with symlinks resolved, the directory of the file.
If used inside an include, the directory of the included file is returned, the function name, the class name, the trait name, the class method and the namespace.
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.
These special constants are case-insensitive.
All these
Magical Constants are resolved at compile time, unlike
Regular Constants, which are resolved at runtime.