The formation of PH4⊕ is difficult compared to NH4⊕ because:

The formation of PH4+​ is difficult compared to NH4+​ because lone pair of phosphorus resides in almost pure s-orbital.Due to this, phosphine lacks nucleophilicity and the donation of the lone pair to a proton becomes difficult.In ammonia, the lone pair is present in sp3 hybrid orbital and can be easily donated to proton.