In some cases, you will need to define global variables or constants who values will not change during the execution of the php code. Php provides you will the ability to define constants using the Define Keyword. The CONSTANTS are defined a differently to normal variables. They start with the DEFINE keyword and do not contain a “$”.

define (”NEWYEARSDAY, “January 1st”);
define (”MONTHSINYEAR”, 12);
define(”NUMBEROFALPHABETLETTERS”26);

Examples of how to display constants.

echo “The number of letters in the alphabet are ” . NUMBEROFALPHABETLETTERS ;

echo “New years is generally celebrated on ” . NEWYEARSDAY;

echo PHP_OS; Displays the current (OS) operating system that PHP is running.

PHP has a few define constants, sometimes refer to as magic 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 with symlinks resolved whereas in older versions it contained relative path under some circumstances.
__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. (Added in PHP 5.3.0.)
__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).
__NAMESPACE__
The name of the current namespace (case-sensitive). This constant is defined in compile-time (Added in PHP 5.3.0).