How would I fix front porch steps where the slab has broken but under the steps it is empty how would I fill this and not spend too much money? What would I use to support the slab?

If you can get under the stairs you might be able to put a one-inch thick board to use as a base and then support the stair with 4x4's in a vertical position to support the stair. I would use treated lumber for all of this.

Just my thought. Hopefully, others will come along to add their opinions.

Your going to have to dig out as much as you can and pour a slab, 4" thick
And then go to the other side and do the same thing, there may be some areas you just want be able to reach underneath it, depending on how wide they are