At 5'11 and <160 lbs you should be concentrating on your diet and getting your routine polished. A couple years ago I was as small as 150 (I'm 5'8). After lurking on AM for a while I realized that it was my diet that was holding me back.
Sign up at Invalid Link Removed and and log your foods and spend some reading here about bulking diets and save the PHs for later when you start plateau...
...I know, that's not what you wanted to hear, but I'm sure others will agree. If you really want to take a SUPPLEMENT to help out there are a lot of PH alternatives out there: Activate, Powerfull, Mass FX, Anabolic Pump, Hyperdrol, BAM, Ecdysteroids, etc. I still say spend your money something like Invalid Link Removed and some good clean foods.
Even if you took some A.A.S/PH and gained 10-15 lbs...well, you're likely to loose a lot of it, especially if your diet is where it needs to be.