How to use constants in PHP

A constant is an identifier (name) for a simple value. As the name suggests, that value cannot change during the execution of the script (except for magic constants, which aren't actually constants). Constants are case-sensitive. By convention, constant identifiers are always uppercase.

<?php
// global
// can not change or undefined
//start with a letter or underscore

/**
* Method 1: using const keyword
*/
const NUM = 15;
echo NUM;
//  NUM = 23 !!!WRONG - can't change value

/**
* Method 2: using define
*/

define('KEY', "VAL", false);
echo KEY;
?>