[IBM]

Which compound is oxidized and which one reduced in the reaction below?
H3PO3 = H3PO4 + PH3

[Babcock_Hall]

Assign oxidation numbers to the atoms.

[IBM]

Assign oxidation numbers to the atoms.

H3PO3 (P =+3) = H3PO4 (P =+5) + PH3 (P= -3)

[Babcock_Hall]

That looks OK to me.

[IBM]

That looks OK to me.

H3PO3 (P =+3) = H3PO4 (P =+5) + PH3 (P= -3)
Could you tell me now which compound is reduced and which compound is oxidized here?

[chenbeier]

Learn definition of oxidation: Oxidation is a release of electrons. What does it mean for the oxidation number? Reduction is a acception of electrons, what does it means for the oxidations number?

[mjc123]

This is not a standard redox reaction, with an oxidising agent and a reducing agent. It is what is called a disproportionation, in which some of the starting material is oxidised and some is reduced. You start with all P(+3), some of it is oxidised to P(+5) and some reduced to P(-3).
By the way, you should try balancing your equation. Not only the numbers of atoms, but also the oxidation numbers, should balance.