exec, backtics and co.

Hello, I have a problem with a php script which contains an exec (identical problem with backtics 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 must 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 the help. Thank you. Matthieu