I had the same problem and used a toilet flange repair kit from a local building supply store. This consisted of two pieces of flat steel which slip under the broken flange and hold the bolts securing the toilet bowl tight to the flange. This worked well in my case where the bolt area of the PVC flange had fractured. In addition, it could all be accomplished from above thereby eliminating the need to open up the downstairs ceiling. It sure made a repair that I imagined being very difficult pretty straightforward. Good Luck!