Download the regular version here, if you don't already have it:
Download PPSSPP on the App Store
Then start it up and click the Buy PPSSPP Gold
button in the app!For other platforms, see here.
PPSSPP Gold is the best way to support the development of PPSSPP, the leading PSP emulator for iOS and other platforms.
You get a nice golden icon, but otherwise it's the same as the regular version.
NOTE: Starting from 1.19, PPSSPP Gold for iOS is an in-app purchase in the regular PPSSPP app.
Unfortunately there's currently no way to transfer your purchase. I'm working on finding a way.