Php :: Actionscript 3.0 -> PHP -> MySQL Security On A Game Application

I am planning on making a game with actionscript 3.0 (flash). However, I am having some security issues on saving user points. To be more specific, read below and you'll understand what's the exact problem. You are starting my flash application. PHP creates a session for your username. Playing for few minutes and reaching 750 points. You click on "Save my points" button. It connects "game.php?points=[]" with your point amount, hence, game.php?points=750. PHP connects to MySQL and does an update/insert query with the username you entered when opening application, and gets the points with $_GET['points'] as you can see on 3.

The issue is, Anyone who could directly browse "game.php?points=999999999999" would have his points saved in the database. I thought about encrypting the points, however, Flash is a client-side application and anyone could change the "points" value with an application like "Cheat Engine". Once they change the points, encrypted points will automatically be generated by Flash.

I also thought about creating a private key for each player on their signup and encrypt accordingly, but it also won't work because once an user change his points with Cheat Engine, flash will automatically encrypt the points with given private key, hence, another useless theory...