In case of both molecules the hybridization of P is sp³ but there is a difference.

In case of PH3 we have a lone pair present on the phosphorus atom. This lone pair pushes the bond pairs downwards due to lone pair-bond pair repulsions which decreases the bond angle in P-H-P atoms, as shown below.

Bond angle in PH3

But in case of PH4+ the lone pair of phosphorus is donated to the H+ (hydronium ion) for making a coordinate bond. Thus there is not any lone pair on phosphorus atom.

As the bond pair-bond pair repulsions are quite lesser than lone pair-bond pair so it gets proper tetrahedral structure.

The structure with least repulsions will have higher bond angle. The structure is as shown below.

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