In Lewis structure of ${\mathrm{PO}}_4^{-3}$ ion the formal charge on the oxygen atom of  $\mathrm{P}-\mathrm{O}$ bond is:

The distinction between an atom's number of valence electrons in an isolated or free state and the number of electrons assigned to it in the Lewis structure is known as the formal charge of an atom in a polyatomic molecule or ion.
Number of valence electrons in the free atoms determines the formal charge of an atom in a molecule or ion.

(Lone pair electron count $+\frac{1}{2}$ bonding electron count) 

Formal charge on each O-atom is as follows: 

$6-\left(6+\frac{1}{2}\ast 2\right)=6-7=-1$