Variables offer a flexible way of storing data because you can change their values and the type of data they store at any time. If, however, you want to work with a value that you do not want to alter throughout your script’s execution, you can define a constant.
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). A constant is case-sensitive by default. Continue Reading