You should remove the leading & in myRequestString and the problem is likely that the correct content-type header is not being sent. Try adding a call to
[request setValue:@"application/x-www-form-urlencoded" forHTTPHeaderField:@"content-type"];
You should also not pass nil for error, so you can see what the client thinks is going on.
Unrelated, but your PHP code is open to SQL injection attacks.