pH affects the equilibrium between the various protonation states of PO4:
H3PO4 NaH2PO4 Na2HPO4 Na3PO4
In the pH 6-8 range, the middle two are predominant. So, depending on which protonation state is inhibiting your protein, pH could have an effect on the concentration of the inhibitory species.
The other aspect of the pH effect is on the protein directly, since any charged residues to which phosphate binds have pKas (Asp, Glu, His, Lys, mainly), such that the distribution of charged and uncharged states depends on pH.