The closet flange on my cast iron pipe is broken, nothing left but a jagged edge. Should I use a pvc flange that fits down into the existing pipe and screw it into subfloor? The problem is that the jagged edge is not at an even heighth all the way around. Thanks Dan.

First, you should take 4" grinder and smooth the sharp high points on the pipe. Yes, you can use the flange that inserts into existing drain and then secure it to the concrete.