How to fix broken toilet flange from over tightened bolt?
You can purchase a toilet flange repair at any big box hardware store.

Remove the wax ring (buy a new one), remove the bolts, and mount this on top of the existing broken flange:


Then reattach the toilet as per normal, making sure not to over tighten.  

Tip, seat the toilet firmly on the wax ring before you tighten the bolts, and use a STANDARD sized wax ring not a jumbo - if the toilet starts out firmly on the floor before you start tightening then you know you're tight enough when the washer starts to bend in just a little.