If you can't use SSL then create an additional "key" parameter ( a good idea anyway ) which the formula is only known to the AppGameKit app and the server side. The key is based on the parameters being sent.
For example you send score.php?score="100"&username="bob"&key="900cpc" - in this example the key is just score * 9 and a shift +1 in the usernames letters. Of course your key system would be more complex.
If the formula for the generated parameters equals the key then that's 99.9% sure it's a genuine post but never assume so. Always validate every step of the way.
If it's sent like score.php?score="100"&username="bob"&key="1234" and validated as incorrect then it's just ignored.
Overall though never ever assume that the data you are getting externally is valid. Verify every little detail before actioning on it.