PHP had the advantage of having built-in resource limits and little contention over locks and other shared resources that meant sloppy coding wouldn't crash your server. Providers could offer low cost hosting plans because you could put a lot of customers on the same server and have very little trouble. It was a big plus that it was open source and didn't have the crazy high license costs of early dynamic web hosting solutions such as ColdFusion, WiTango, Netscape's application server, etc.