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; ?>
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.
|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.|