For H 3 PO 4

Formal charge on P in H3PO4is

Here shouldnt the total no. of shared electrons be 10 and a double bond between one O and P so as to satisfy the valency 5 of phosphorus??

Formal charge is the charge assigned to the atom assuming that electrons of a covalent bond are equally shared between two atoms regardless of their electronegativity. 
Formal charge = No. of valence electrons in a neutral atom - 12(No. of electrons in covalent bond) - No. of electrons in lone pairsNumber of electrons in covalent bonds are the eletrons which are shared. This can be directly stated as the number of covalent bonds. So, we can simply write the formula as:Formal Charge = No. of valence electrons in a neutral atom-(No. of covalent bonds)-(No. of electrons in lone pairs)

Beyond the second row in the periodic table, the octet rule is not obeyed with satisfying regularities. However it remains a useful rule. Atoms of heavier elements do more than obeying the octet rule. Some of them show a surprising ability to bind more atoms than would be predicted by octet rule. 
For example: P and S, in PF​5 and SF6. In these all the valence electrons are shared which means P shares 10 electrons and S shares 12 electrons which violate the octet rule. 
This violation occurs because of the presence of vacant 3d orbitals which can be used for the electron pair sharing means extra bonds are possible. 
This whole thing was explained by Atomic Theory that is Theory of Atomic Structure. 

So, Formal charge of P in H3PO4 is: 
No. of valence electrons on P = 5
No. of shared electrons= 8 or No. of covalent bonds= 4
No. of lone pairs= 0
Formal charge on P = 5-4-0 = +1
 

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