PHP Script to optimize and checking security holes on it.

as topic says, we have few files written in PHP. running on server. we need to check the script security holes and optimize it for loading and running faster,, we believe you might modify some piece of php lines. the project is a small scale one, used to let subscribers download files per their credits or payments. for script optimization we know that some optimizations carried by server it self, so we prefer to get a freelancer who is talented in optimization from both Server side and php script side BR/