I have an exterior drain in a stairwell that has broken out of the cement. It had a stand pipe. All I can find about 7" below the cement is a cast iron p-trap. and remnants of clay tile in the excavation of the dirt. The ID of the trap is 5" so I will have to nestle a short length of pipe and join it to a new drain. Can I use PVC? How would I join it to the p-trap? I’ve just carefully excavated and cleaned out the trap to make sure it was running to the cistern in the back yard.

Pleas give us more details, What size is the raiser coming out of the trap? Is it cast iron also? Since you already are trapped perhaps something like this would work. (see image). You will have to transition to PVC but if you give me enough information we'll work it out. Cheers. Tom

From the P trap it used to be tile, but that has broken away Now there is nothing.
See pictures

Install a new bell trap and replace the old tile with perforated plastic drain pipe. (see image) good luck, Tom

Sorry, I must not have been clear, I need to know how to come up to a drain from a cast iron ptrap.
The piece of tile I mentioned was the riser. What should the riser be, to fit into a 5" ID collar of the P-trap?
Do I need to get a gasket something like a 5" donut?