TUTORIAL: Defining a Constant with PHP
From the blog:
Defining a constant in PHP can come in handy when working with a value that will be used throughout your script. What is a constant exactly? According to PHP.net, “A constant is an identifier (name) for a simple value“. One important thing you need to know is that, like its name states, a constant cannot be changed. That is pretty much the main difference between a constant and a variable.