But are you really keen to make a PHP dynamic webpage application where each page imports some database function/credentials and uses them to render html?
Can you keep the behavior of fluent userflow (e.g. menu not rerendering) that way? Only with minimal design.
When in 2006 most webpages had an iframe from the main content, an iframe for the menu, and maybe an iframe for some other element (e.g. a chat window), it was fine to refresh one of those or have a link in one load another dynamic page. Today that is not seen to be very attractive and to common people (consumers and businesses), unattractive means low-trust which means less income. Just my experience, unfortunately. I also loved that era in hindsight, even though the bugs were frustrating, let alone the JS binding and undefined errors if you added that...
If you don't care enough to figure it out, then you don't care enough to make it secure and that leads to very very bad time in modern largely internet-centric world.