Let's say you want to define a variable that cannot be changed after it is defined. You'd like to use this variable for a configuration option, message, or something else of this sort. You can do this using constants. Constants are a programming structure common in many programming languages. It is a scalar variable that cannot be changed once defined. In addition to not being changed, it is in the global scope automatically.
Here is how you would define a php constant
It is common to define a constant name in all caps, this makes it easier to identify it's use as a constant. You can use this constant in your programming pretty easily
echo 45 * MY_CONSTANT;