PHP is a procedural language, which means that it generally starts at the top of the script and runs to the bottom. When it hits the bottom, it's done until the next web page is requested. Of course, you can use functions and include files, but ultimately it starts at one point and ends at the last line of code on the page. PHP has some support for objects and classes, but it's not inherent and in my opinion, it's cumbersome and feels kludged.
Thanks a lot