Every acid base reaction will produce complement conjugates...
Now, what does that mean... I'll show you

base + acid  ⇒  conjugate acid + conjugate base

PH₂^– + H₂O  ⇒  PH₃ + OH^–

in other words, H₂O has to be the acid


Another way of looking at this


PH₂^– goes into the reaction with a negative charge, making it basic
it comes out as PH₃, an overall zero charge

In other words, it received a H⁺ from the water molecule

H₂O = proton donor = acid


Either way you slice it, in this reaction, water is the acid