## 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 five 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. Since PHP 4.0.2, `__FILE__` always contains an absolute path whereas in older versions it contained relative path under some circumstances.
`__FUNCTION__` | The function name. (Added in PHP 4.3.0) As of PHP 5 this constant returns the function name as it was declared (case-sensitive). In PHP 4 its value is always lowercased.
`__CLASS__` | The class name. (Added in PHP 4.3.0) As of PHP 5 this constant returns the class name as it was declared (case-sensitive). In PHP 4 its value is always lowercased.
`__METHOD__` | The class method name. (Added in PHP 5.0.0) The method name is returned as it was declared (case-sensitive).

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