As you already know, P is being oxidized and it is the reducing agent in this reaction, as Chlorine is being reduced and it is the oxidizing agent for the reaction, but for oxidation number, there is one crucial information that will give us the answer:
Oxidation number for halogens will 99% of the times be -1, since they have 7 electrons in the valence shell and they need 1 to stabilize.
Cl2 is stable as the 2 chlorines are sharing electrons, but Cl alone will have a -1 charge, and Cl with other elements making up a compound will also have a -1 charge
Therefore if we have 5 Chlorines with P, and each Chlorine has a -1 charge, therefore Phosphorus will have a +5 charge in order to balance and stabilize charges, and this is experimentally proven, as P has oxidation numbers = -3, +3 and +5.
Oxidation number change
P = P^5+
Cl = Cl^-