PHP would deal with uploaded files by itself and write them correctly and directly to disk (unlike some Java implementations like Nexus which buffers in RAM, you can guess what happens).

As for ordinary POST/GET parameters, it stores them in a string aka a byte stream which you can then post-process e.g. by translating to UTF-8 based on the browser encoding header. So basically the only way to shoot yourself is if you're doing substr and friends on user input instead of using the mb_ variants.