Starting at the upper right corner and moving clockwise we can make 4 resonance structures that expand the P octet to 10.  In all of these the P has a formal charge of 0 and one oxygen is also 0.  The last resonance structure expands the P octet to 12, but is not a very good structure because the less electronegative P has a more negative formal charge then some of the O.  The best resonance structures are the four at the bottom.