The oxidation number of phosphorus in \(PO_4^{3-}\) is calculated as follow;

\(PO_4\) = -3

but oxidation number of oxygen = -2

P + (-2 x 4) = - 3

P - 8 = -3

P = -3 + 8 = +5

\(\therefore\) the oxidation number of phosphorus in \(PO_4^{3-}\) = + 5.