PHP Constants

PHP constants are name or identifier that can't be changed during the execution of the script. PHP constants can be defined by 2 ways:

PHP constants follow the same PHP variable rules. For example, it can be started with letter or underscore only.

Conventionally, PHP constants should be defined in uppercase letters.

PHP constant: define()

Let's see the syntax of define() function in PHP.

name: specifies the constant name
value: specifies the constant value
case-insensitive: Default value is false. It means it is case sensitive by default.

Let's see the example to define PHP constant using define().

File: constant1.php

<?php
define("MESSAGE","Hello trainingtrains PHP");
echo MESSAGE;
?>

Output:

Hello trainingtrains PHP

File: constant2.php

<?php
define("MESSAGE","Hello trainingtrains PHP",true);//not case sensitive
echo MESSAGE;
echo message;
?>

Output:

Hello trainingtrains PHPHello trainingtrains PHP

File: constant3.php

<?php
define("MESSAGE","Hello trainingtrains PHP",false);//case sensitive
echo MESSAGE;
echo message;
?>

Output:

Hello trainingtrains PHP
Notice: Use of undefined constant message - assumed 'message' 
in C:\wamp\www\vconstant3.php on line 4
message

The const keyword defines constants at compile time. It is a language construct not a function.

It is bit faster than define().

It is always case sensitive.

File: constant4.php

<?php
const MESSAGE="Hello const by trainingtrains PHP";
echo MESSAGE;
?>

Output:

Hello const by trainingtrains PHP