I'll try to explain this by comparing this with the case of NH3.
N is a better electron withdrawing group than P. So in the case of NH3 the electron cloud is more closer to N. And therefore the repulsion in the bonds causes the 3 H atoms to be farther away from each other resulting in a bond angle of about 109°. But in the case of PH3 the electron cloud is farther away from the central atom(P) than in the case of NH3. So the bond pair - bond pair repulsion is comparatively lesser, causing the 3 H atoms to move closer together to an angle of almost 90°, resembling the px, py, and pz orbitals, as a consequence it's lone pairs are present in almost pure s - orbital. So it is is said that PH3 can be viewed as unhybridized.