Are you using only an index.php and loading new content in it or do you have seperate pages for example, about.php/contact.php?
If you’re only using an index file i’d suggest using $_GET variables to add an active class to your menu.
If you’re using seperate pages you can define a variable before including your header that tells it which page it is.
After a quick google I found these examples:
I hope this helps, if not let me know!