The formation of ${{\mathrm{PH}}_4}^{\oplus }$ is difficult compared to ${{\mathrm{NH}}_4}^{\oplus }$ because:

The formation of ${\mathrm{PH}}_4^{+}$​ is difficult compared to ${\mathrm{NH}}_4^{+}$​ 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.