exec, backticks &Co.

Hello, I have a problem with a php script which contains an exec (same problem with backticks and similar operators). Some programs are simply not executed. For example, having a simple 'hello world' program in c++ will work fine, but having a program that writes 'hello world' to a file won't work. My question is "What are the requirements a binary much match to be correctly executed?" I assume there are some stuff about - permissions... - location in the directory tree... - ??... I don't know where to find this information, so if anyone has suggestions or pointers, I would really appreciate. Thank you. Cheers, Matt