There are a number of libraries for highlighting PHP code that have varying reliability and are differently challenging to install.
If you don't have a lot of requirements for highlighting, you can also use the built-in PHP functions directly.
Basic…

Highlighting PHP code syntax with highlight_string()

<p>PHP contains a surprisingly large number of handy features of all kinds, which are especially appreciated by advanced developers. This page provides a basic overview and examples of their use.</p>


Handy features