BasicallyNH3 is a base due to the lone pair of electrons present on the Nitrogen atom which is available for donation. On the other hand PH3 is also a base but its basic character is less than that of NH3 because the nitrogen in phosphorous have lone pair of electrons that are not readily available for donation.
Also the P-H bonds are weaker than N-H bonds, so there’s not as much compensation for protonating PH3. This is a result of the inert pair effect, seen in heavier elements.