In PO43− ion, the formal charge on each oxygen atom and P-O bond order respectively are

Bond order = Total number of bonds between atoms  Total number of resonating structure  Bond order =5​/4=1.25In a given resonance structure, the O atom that forms double bond has formal charge of 0 and the remaining 3 O atoms have formal charge of -1 each.In the resonance hybrid, a total of -3 charge is distributed over four O atoms. Thus the formal charge of each O atom = −3​/4 = −0.75.