Shortcut to flip a layer horizontal/vertical

Although it's an extra step, you could drag in a guide and snap it to one edge of the layer. Then, Alt+Drag the opposite edge all the way over, until it snaps to the guide. You could probably make a script that would place the guide automatically (and possibly clear it when done).

Thanks tons!

How to make a flip canvas button in photoshop hotkey How To Draw Tutorial - YouTube  this show you how to do it with the push of one button

Thanks Dave, and Randy, and C. Pfaffenbichler, for your replies. I always assumed that I didn't know that obscure "secret" way, but evidently there isn't a secret way. Having considered the matter, the assigning of a custom shortcut is probably simplest way to go forward. Many thanks to you!

I do not know of a keyboard shortcut but you can add a negative sign in the tool bar option window. It will than flip transformed selection to exact size.