There is none. But try putting Ditto and Manaphy in the daycare together...
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You can breed manaphy and rotom, but you need a ditto to do so. Put one of the legendary Pokemon in the day care and you get an egg. But if you put manaphy in with ditto you get a phione, to get another manaphy you need an action replay and the code to go with it.
There is no legitimate cheat code to get a Manaphy egg. You would have to hack the game with something like an Action Replay to get one.
There is absolutely no way to hatch it.
Pokémon Heart Gold is a popular video game. The action replay code for an instant egg hatch for the U.S. version of this game is 12029318 000020FE.