Servers are cheap, programmers are expensive and bugs can kill you.
I suggest going with whatever gives you clean and correct code faster. I wouldn't suggest PHP for a new project. If you already have a big codebase written in PHP, you should keep it, but by "keep" I mean maintaining and cleaning up, not only adding to.
As for MySQL, it's good enough unless you get too successful - then it becomes painful to keep it going. Mostly any RDBMS has this problem. Again, for a new project, I'd go with PostgreSQL, but that's more a matter of taste.