what is drago rule? can one know the hybridisation of PH3?

Drago’s rule:

For any Group in general, the formula for calculating the number of hybrid orbitals is group number of central element + no.of atoms surrounding the central element(except oxygen)-charge on the central element(consider the charge including its sign) whole divided by 2.

Hybridization of PH3 is sp3 and the bond angle is 93degrees.This is because Phosphorous(P) has 3 bond pairs and a lone pair that gives it a SP3 hybridization .