In PH3, phosphorus (P) exhibits an oxidation number of -3.

To determine the oxidation number, let's assume the oxidation number of phosphorus (P) as "x". Each hydrogen (H) atom carries an oxidation number of +1. Since PH3 is a neutral molecule, the sum of oxidation numbers must equal zero.

1(x) + 3(+1) = 0

x + 3 = 0

x = -3

Therefore, the oxidation number of phosphorus in PH3 is -3.