The formation of  $P{H}_4^{+}$  is difficult compared to $N{H}_4^{+}$  because:

According to Drago's rule lone pair on phosphorus resides in almost pure s-orbital, hence due to non-directional nature, its overlapping tendency is greatly reduced in comparison to a lone pair present in hybrid orbital, which is directional as present in  $\overline{N}{H}_3$.