Ok, now that there is an intro....
Back fill and pack the hole to about the same debth as the existing concrete. Tamp it well.
Drill holes horizontally into the existing, then insert rebar. Try to "crosshatch" the rebar and tie them together with tie-wire. Don't put them too close together though, you should have squares open at least 2" x 2" to allow the concrete to adhere properly. Pour in a properly prepared redi-mix to the desired floor level and smooth it out, preferably with a proper trowel, but a 2x4 piece will work nearly as well.
Then look at the trap that is sitting on the floor next to you, curse, and try to get all of the stuff out before it sets! Install the trap and repeat.