What is a hook in PHP?

You probably couldn't find anything because PHP doesn't have a concept of hooks in the first place.

Hooks are a kind of function which you can plug (or hook) to an existing system to extend its functionality. They aren't specific to the PHP language or to any system. They may also be called plugins, add-ons or extensions.

Now, while PHP doesn't have a concept of hooks, it does allow you to compile extensions together with the PHP core to gain added functionality for use in your scripts. There are plenty of PHP extensions bundled by default. This is an example of what I described above.

Yeah, hooks aren't native PHP methods.. they're used to extend functionality from a framework's core.

Codeigniter Hooks