The following has been excerpted from the PHP manual.
PHP provides a large number of predefined constants to any script which it runs. There are seven 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:
The current line number of the 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 with symlinks resolved whereas in older versions it contained relative path under some circumstances.
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. (Added in PHP 5.3.0.)
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.
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.
The class method name. (Added in PHP 5.0.0) The method name is returned as it was declared (case-sensitive).
The name of the current namespace (case-sensitive). This constant is defined in compile-time (Added in PHP 5.3.0)