PHP Rest Web API multiple functions in single file

I'm new to the PHP/Web world so excuse me if this is a simple question.

I have seen many links on how to make a web-api using PHP, and all of the examples would put each "function" in a different file. I once helped my friend develop an asp.net C# web api where a single file would contain multiple functions.

Can this be done in PHP? i.e can a single PHP file contain multiple functions that a web-api can call? If so can you kindly give me an example of how it is done, or refer me to a link where this is explained?

Thank you for your time.