Answer
Hybridization of phosphorus in the phosphonium ion, $P {H_{4}}^{+}$ is s$p^{3}$.
Work Step by Step
Both $P {H_{4}}^{+}$ P$H_{3}$ have sp3 hybridization state for phosphorous.In $P {H_{4}}^{+}$, all the four orbitals are bonded, whereas in P$H_{3}$, there is a lone pair electrons on phosphorous in $P {H_{4}}^{+}$, the H-P-H bond angle is tetrahedral angle of 109.
