Not any more nowadays. I am the original author of Photon and today PHP is extremely robust and you can catch all the exceptions/errors (outside the ones which are making your code invalid, but hey, do your homework). The current PHP 5.3 is also wonderful at not leaking memory. So you get the speed of bare PHP with a framework (because nothing is reloaded, the framework can really load once, reuse many times).

On a small system, a standard HTTP request through Mongrel2, to the framework and back is about 2ms (including 1ms of network latency between the hosts, data from my production monitoring). This is the "hello world" latency. It makes using PHP very fun and very fast. Also, a single PHP process running in the background uses about 12MB, it means that with 12MBx3 + 10MB Mongrel2, you can serve 1000's of users with nearly no load on your system (if you push the load only when needed at the DB level). Tracing a PHP process, you can run it without system calls at all (only gettimeoftheday for logging).

But, this is cutting edge, so, you need to be open minded and ready to dig in the code (less than 25k SLOC anyway I think).