WebReference.com has posted part three of their "Building Blocks" series - a look at data types, literals, variables, and constants.

In part three, they focus more on the last type of "block" in their list - the constant:

Some real-world constants, such as pi, the speed of light, the number of inches in a foot, and the value of midnight, are values that don't change. PHP not only provides its own predefined constants but lets you create your own. Using constants makes it easy to write and maintain your programs.

There's talk of the define() and constant() functions and a mention of some of the predefined and "magic constants" as well.