psx4droid did not stay long in the market because it was taken out before Xperia Play was officially released. However, its latest version was made available at the website of the developer of the emulator. It was even for free.
To use this emulator, find a full disc images and PSD BIOS file saved to a dedicated PSX ROM folder meant for the emulator alone. However, these things may took a backseat once you start with Resident Evil or WipeOut.
If you opt to use the emulator using a handset, such as my Desire HD, be it a medium to higher quality device, it works well on most games. Only minimal menu programming was necessary while the framerates run well.
The game can be played comfortable by moving around the on-screen controls to give way to the movement of the D-pad placed away from the upper left part. However, its top left position is a default so you just need to bear with while trying to ignore the vibrating buttons, a reminder from those days without touch technology.
Once your emulator is on, it would be great to activate some advanced options. For instance, exchanging framerate for anti-aliasing would be a good try.
PlayStation games may be more ancient than console games due to the basic of all basic 3D graphics before. However, this is all fine by the Sony loyalists, who are willing to support the emulator.