8. Constants and Predefined Constants

Constants: A constant is an identifier for a simple value. A constant value cannot change during the execution of the script. By default, a constant is case-sensitive. constant identifiers are always uppercase. A constant name starts with a letter or underscore. If you have defined a constant, it can never be changed. To define a constant you have to use define() function

<?php
	define("GREETING", "Welcome to Constant area");
	echo GREETING;
?>

Predefined Constants:
PHP features a large number of built-in, predefined constants holding various useful values. Some of these are always available. For example, the PHP constant M_PI holds the mathematical constant Pi. The following code displays this value:

<?php
	echo M_PI;
?>

Another useful predefined constant is PHP_VERSION, which holds the current version of the running PHP engine:

<?php
	echo PHP_VERSION;
?>
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