First, consider the overall charge of the ion. This is -3.
We know that the overall charge is the sum of the charges of the constituent atoms. Moreover, oxygen exhibits a -2 charge usually. We will now use this information:
Sum of charges:
P + 3O = -3
P + 3(-2) = -3
P = +3
Therefore, phosphorus has a charge of +3 in the phosphate ion.