PHP Programing language

A constant is an identifier for a simple value. A constant value cannot be modified during the execution of the script. By default a constant is case-sensitive. It can be defined by using define() function or by using keyword const outside a class definition.

<?php
define("Student_name", "Ankur");
echo Student_name;
?>

Output:

Ankur

Valid and Invalid constant names:

<?php  
//Valid constants name  
define("student_name", "Ankur");  
define("class", "V");

//Invalid constants name  
define("2NO_OF_STUDENTS", 60);  
define("__Total__", "total value"); 
?>

PHP Magic constants:

PHP provides a large number of predefined constants to any script which it runs.

SNo.	Name	Description
1	__LINE__	The current line number of the file.
2	__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.
3	__CLASS__	The class name. The class name includes the namespace it was declared in (e.g. Foo\Bar). __CLASS__ works also in traits. When used in a trait method, __CLASS__ is the name of the class the trait is used in.
4	__TRAIT__	The trait name. The trait name includes the namespace it was declared in (e.g. Foo\Bar).
5	__METHOD__	The class method name.
6	__FUNCTION__	The function name.
7	__NAMESPACE__	The name of the current namespace.
8	__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.