There are two situations when I would not recommend PHP for use in a web application. Firstly, as already mentioned, if you desperately need all the performance you can get, you should be using C or C++. Both languages are unwieldy on the web, but they offer superior performance. Even in this situation you might want to consider simply writing your C code as extensions to PHP.
Secondly, if you are adding to a system which already has a lot of code already written in another language, then coding parts in PHP will only make your maintenance job more difficult, so you are likely to find it easiest in the long run to keep using the old language.
Author's Note: It's important for me to make the point that, although PHP is thread-safe, its libraries may not be. If you don't know what thread-safe means, you don't need to worry about it!