Yup 99% chance its in your PHP. May even have a dead-loop, or at least poorly optimised code. Put in echos as Zuluru says - and I like to put in timings, and comment out chunks of heavy code, slowly re-introduce so you can find the bottlenecks. In fact, as Zuluru said, start with nothing but Hello World and work your way up!
Also, politely, learn how your laptop works! I would guess you have a HP and it’s got a function override that hijacks your function keys (I hate those things!) so that F12 has been repurposed.
You can also do Google searches on other ways to open the dev tools, which lead me to this article Chrome DevTools | Google Developers
because you will need a decent client-side debugger sooner or later!