This page explains constants

Constant are like variables declarations, but are written differently and cannot be changed or undefined. For example, declaring a varible looks like this: $example = 1. But a constant uses define ('x',x). The first x is used to name the variable while the second is used to define it.
Example: define ('tax_rate', .07) define ('shippingfee', .01). Using the constant ('cost', 30) this would be the output using the respective constants.
cost * tax_rate: 2.1
cost * shippingfee: 0.3