Answer
$PH_3(aq) + H_2O(l) \lt -- \gt P{H_4}^+(aq) + OH^-(aq)$
$K_b = \frac{[OH^-][P{H_4}^+]}{[PH_3]}$
Work Step by Step
The ionization equation should describe the transfer of a proton from a water molecule to the base, therefore, the base and the water are the reactants, and the protonated base and a hydroxide ion are the products.
The ionization constant expression is based on:
$K_b = \frac{[Products]}{[Reactants]}$
* The water doesn't appear, because it is the solvent, and its concentration doesn't matter.