What is the hybridization of P atom in phosphate anion ?

Phosphate ....

What makes you think that there is any hybridization?  Hybrid orbitals were "invented" in the 1930's as a way of explaining the geometry of mostly organic molecules.  It seems to be a useful model for some compounds, particularly if the central atom is in period 2, but for elements in periods 3 and up, not so much.

To use hybridization we first need the geometry and then the hybridization will follow.

linear ..... sp

trigonal planar ..... sp2

tetrahedral ..... sp3

-- And by the way, there is no sp3d or sp3d2 hybridization.  Chemists have shown there is no d-orbital participation in the bonding in hypervalent molecules (trigonal  bipyramidal and octahedral).

The simplest way to represent PO4^3- is with a tetrahedral structure...

......O

.......|

O = P − O

.......|

......O

Each singly bonded oxygen has three lone pairs, and a formal charge of -1.  This will make it consistent with the -3 charge on the ion.

Since it is tetrahedral, and you are apparently looking for some kind of hybridization, then we could say that phosphorous exhibits sp3 hybridization.