This is a Little Hombrew game called ZombieInf2. Basically your just running to the right shooting zombies and jumping lol, Bringing back jump and shoot man heres the download and how to install below.
Step 1: Download and extract the ZombieInf2.RAR file and you will see the ZombieInf2 folder inside there.
Step 2: Connect your PSP into USB mode and go to the ms0:/PSP/GAME folder, or for PSP go if you use HDD ef0:/PSP/GAME folder, and copy the ZombieInf2 folder to your GAME folder.
Step 3: Exit USB mode and there ya go you can now kill zombies by scrolling to the game on the XMB.