How can I change PHP constants? [closed]

I am working on making my own custom CMS in PHP by hand and I have a few constants I have defined. Is there an easy way to modify the constants?

I was thinking about using something like fopen() and then changing it, but I have never used the filesystem functions.

Answer

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. By convention, constant identifiers are always uppercase.

The name of a constant follows the same rules as any label in PHP. A valid constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thusly: [a-zA-Z_x7f-xff][a-zA-Z0-9_x7f-xff]*

From: Constants (PHP mamual)