Constant

Only scalar data (boolean, integer, float and string) can be contained in constants.

Differences between constants and variables are

• There is no need to write a dollar sign ($) before a constant, where as in Variable one has to write a dollar sign.

• Constants cannot be defined by simple assignment, they may only be defined using the define() function.

• Constants may be defined and accessed anywhere without regard to variable scoping rules.

• Once the Constants have been set, may not be redefined or undefined.

Predefined constants

__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).