a) try your new code by saving in your editor, and hitting reload in your web browser.
b) it's very approachable. People who only know HTML and CSS can be expected to do a little bit of PHP work to integrate their changes. If you setup the right network mounts, they just need to edit files and reload (see a)
c) it's not super high overhead at runtime. If you're not using a framework, and you don't build up a crazy object hierarchy, it's not too hard to get your page out with about 10 ms of overhead beyond data fetching. For very simple webservices (fetch data, possibly from multiple sources, and do a little formatting for the consumer), I was able to get the overhead down to 2ms. You can certainly do better with other languages, but you can usually get better throughput improvement by working on getting data quickly. Btw, all the frameworks are terrible; many of them add 100 ms to the page just for the privilege of loading the includes; PHP is a framework for web programming thank you very much.
d) cleanup; you don't have to worry about it. If you don't do anything weird (c extensions, with non-preferred malloc), at the end of the request, everything is thrown away.
That said, there are plenty of things PHP isn't good at: I wouldn't run a long running process in PHP; and multithreaded PHP sounds like a bad idea.