Bond order of P-O bond in ${\mathrm{PO}}_4^{3-}$ is:

Phosphate ion corresponds to resonance and the bond order of P-O is calculated as:

$\mathrm{Bond} \mathrm{order}=\frac{\mathrm{Total} \mathrm{number} \mathrm{of} \mathrm{bonds}}{\mathrm{Total} \mathrm{number} \mathrm{of} \mathrm{resonating} \mathrm{structures}}$

There are four resonating structures of phosphate ion and each structure has five P-O bonds:

$\mathrm{Bond} \mathrm{order}=\frac{5}{4}=1.25$