PHD Flopper is a perk available for purchase for 2000 points. It makes you invulnerable to explosive damage, as well as falling damage. In a situation where you normally would've taken damage from falling, you will instead cause a large explosion, damaging surrounding zombies.
The PHD Flopper perk-a-cola machine
A player drinking the PHD Flopper perk-a-cola
The explosion resulting from falling from a high height with phd flopper
Trivia-
Ph.D flopper may not explode if the player is standing up or crouched when landing from a high fall. The most assured way to use Ph.D flopper is to use it when diving.
Ph.D flopper eliminates the occasional Infinite blast radius on grenades and betties.
The strength of the explosion caused by Ph.D flopper depends on how long the player fell before hitting the ground. It can also do extra damage with high gravity situations.
Pro's- Nice price, eliminates fall damage, turns the player into a weapon.
cons- Doesn't always do it's job. (glitch or coding error.)