Open Terminal.app and type ifconfig
and then hit return. You should see something like this. I've highlighted the IP
Then visit that IP in your web browser. You should see the same thing that you see when you visit localhost
If your phone and computer are on the same wifi network, you should now be able to access your localhost from your phone.
My iPhone is still not connecting to the ip address provided when I tried this. I suspect it's in my config or vhosts setup. Mystery world for the likes of me.