Here's the walkthrough I found.
Some of the PoP: The Forgotten Sands game boxes have a little red stamp that says "BONUS: Full Game Prince of Persia 1992" There's a bit to a gimmick to that though. You need to unlock the game to be able to play it.
Now, to obtain the Classic PoP, you need to finish Level 1: The Stranded Castle and go on to Level 2 where you kiss a statue for the first time in the game. In that particular area, there are a couple water falls. Check behind each of them until you see the button Z appear on the screen. When it does, press Z. Then, you should see the prince pick up a green bottle and drink it. This will unlock the Classic Prince of Persia.
To play the game, go back to the menu, select your profile, go to Extras, Maps, then scroll down until you see Prince of Persia Classic.
