Hybridisation of P in PH₃ is sp³. Out of the four sp³ hybrid orbitals, three are involved in bonding with the three H-atoms and the fourth one has a lone pair in it.

PH₃ when combines with a proton, it forms PH₄⁺ in which the lone pair is absent. So, here all the four hybrid orbitals are involved in bonding.

Now; the lone pair- bond-pair repulsion is stronger than the corresponding bond pair-bond pair repulsion; thus, the tetrahedral shape associated with sp³ bonding in PH₃ is changed to pyramidal and due to the absence of lone pair in PH₄⁺, there is no lone pair-bond pair repulsion. Hence; the bond angle in PH₄⁺ is higher than the bond angle in PH₃.