How is phosphate stable disobeying octet rule? Doesn't the phosphate now have 10 electrons mutually shared between the oxygen atoms? I am unable to understand how the phosphate bond would be formed if phosphate does not obey the octet rule. 

Thank you!

Reference: http://www.chemguide.co.uk/organicprops/aminoacids/makedna.gif
The octet rule is just a rule. Once you leave the 2nd row of the periodic table, the octet rule doesn't apply. Further reading: https://en.wikipedia.org/wiki/Hypervalent_molecule