I have a hose bib outside that comes up and out of the concrete
walkway. The 1/2" PVC pipe comes about 3/4" of an inch out of the
concrete before it meets with a coupling, then about a foot of 1/2"
pipe, and then the hose bib itself.
I've got a small leak between the upper part of the coupling and the
foot of pipe.

I agree with the other posters, exposed pvc pipe isn't going to hold up
very well outdoors, either due to sun exposure or just the stress of people
bumping the hose bib and breaking the fragile PVC.

If the pipe is fairly close to the edge of the walkway, I would dig up the
line under the walk, and then use a masonry bit to widen the hole. Run a
metal pipe down through the concrete, then make a new connection with the
PVC line underground.

Otherwise, relocate the bib somewhere else and abandon the one in the walk,
or tear out a section of walk and rebuild it.

Anthony