I've been developing php apps since 3.0.5 or so, and I can definitely say php is rock solid stable. The only time I was ever able to bring my server to it's knees (it eventually would recover) was to throw ugly queries at postgresql 6.5.x. Since upgrading to postgresql 7.0.x, I have not had one instance of the server going offline due to load or bad code.
PHP/Apache/Unix is a rock solid combo. Your choice of database is far more critical of a decision.