Encryption/Decryption between Flash & PHP

Consider I made a game. Everytime someone plays it , it sends the highest score to a PHP script.

This high score update can be hacked if someone decompiles the actionscript and finds the variables and the query. So which type of encryption I have to use in this condition?
And how?

What if can use Amayeta SWF encryption 5.0? Is that alone helps?

Earlier I tried something like this for an online flash with registration.
Flash sends variables into a php script(this script was in another domain) which encrypts the data and return.
Then flash will receive this encrypted data and send to the main php in the same domain.