Answer
$PH_3$ is a Lewis base.
Work Step by Step
Drawing the Lewis structure, we can see that, the central atom has a lone pair of electrons, therefore, this compound can behave as a Lewis base.
For example, when reacting with water:
$PH_3(aq) + H_2O(l) \lt -- \gt P{H_4}^+(aq) + OH^-(aq)$
The lone pair is donated to create a bond between the molecule and one hydrogen from the Lewis acid: $H_2O$.
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