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 nine 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. All these "magical" constants are resolved at compile time, unlike regular constants, which are resolved at runtime. 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 with symlinks resolved. 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__). This directory name does not have a trailing slash unless it is the root directory.
__FUNCTION__	The function name, or {closure} for anonymous functions.
__CLASS__	The class name. The class name includes the namespace it was declared in (e.g. Foo\Bar). Note that as of PHP 5.4 __CLASS__ works also in traits. When used in a trait method, __CLASS__ is the name of the class the trait is used in.
__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.
ClassName::class	The fully qualified class name. See also ::class.

See also get_class(), get_object_vars(), file_exists() and function_exists().