If you're on the same lan it's as simple as going to the IP address of the local server on your phone.
I recently tried exactly this. Running MAMP on my MacBook pro, went to 192.168.0.5 (my MacBook's IP address) on my iPhone while connected to the same wifi as the MacBook and there was the MAMP localhost page.
The only problem is (as far as I know) there's no way to edit /etc/hosts on an iPhone (although I imagine you can do it on Android), so it has to be IP based.