Phosphorus does not form pπ-pπ bonds but how could it form dπ-pπ bonds?
My doubt is then how does phosphorus form dπ-pπ bonds? 


Poorly. Influence of the $d$-orbitals on the chemistry of the $p$-block is minor. The current belief is that 'double' bonds are actually dative bonds in most cases. 

See the similar cases of chlorine and sulfur 

How does chlorine form more than 1 bond?

What is the chemical structure / hybridization of sulfuric acid?

For phosphorus it isn't that bad, but still bad

http://link.springer.com/article/10.1007%2FPL00010730