PHP was simple to use, and to understand its logic, because it was embeded into HTML itself if you want to. No need to write views, controllers, templates, anything. And most of the shared hosting (paid and free) supported it.
[1]Includes were like magic back then because if you had a heavy content website with a lot of inner pages (fansites), updating the navigation sidebar was a pain in the ass. Originally they were made with framesets, later iframes (with no borders, but if the content changed, a scrollbar appeared). When I (and my friends) started to use PHP and be able to use includes, it was awesome.
(Edited to fix formatting)