WWE's WrestleMania Kickoff was nothing short of bonkers ... with The Rock straight-up slapping Cody Rhodes in the face in a wild on-stage confrontation!!
The whole event went down in Las Vegas on Thursday ... with Roman Reigns addressing the crowd and announcing he will face off against Dwayne Johnson in Philadelphia -- claiming it's his decision and there's nothing Cody can do about it.
Rock then showed up and hopped on the mic .