The hybridisation of phosphorous in PO _ 4 ^ 3 - & is

The structure of ${PO}_{4}^{3 -}$ is as follows

In the given molecule, The P atom have 5 valence electrons. The hybridization can be determined by the number of sigma bonds and the number of lone pair.

As phosphorous central atom forms four $\left(4\right)$ sigma bonds and has no lone pair of electrons. It uses four hybrid orbital and thus, hybridisation of $p$-atom is ${sp}^{3}$.