Constants
Constants in PHP are named values that cannot change during the execution of a script. They are case sensitive like variables but are defined separately.
You can use the const keyword to define a constant.
<?php
const NAME = 'Ryan';
The convention in PHP is to use an uppercase name for all constants. You're also allowed to use underscores and numbers.
define()
Another method for defining constants is the define() function. This function accepts the name of the constant and it's value.
<?php
define('NAME', 'Ryan');
It's highly likely you'll see this in PHP code, however the more modern approach is using the const keyword from above.