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How can I prevent cheating on global highscore tables?

So, I create a game in Flash and I want to keep a hi-score table for people to compete in.

After a player has finished playing a game, the game needs to transmit the score (& player's name etc) back to a central server, to store the results in a database, probably via a PHP web page for simplicity.

All this is fine, but at this point nothing is stopping some 'clever' individual from sending a their own customised message directly to the PHP page and entering their own values directly into the hi-score tables, therefore cheating the data without even playing!

So, what can I do to ensure that the data being sent is only validated if it comes from the game itself?