I have just done this. The way it works is this:
I have an Arduino Uno and an Arduino Wifi Shield, also a internet hotspot through an Android phone.
The Arduino connects to the hotspot, reads all pin values, sends the information to my server/php. Like this: http://myserver.com/stringdata
After the server has read the data sent, it does calculations based on it and replies with a body message in form of a string. Like this: echo $returnstringdata.
The Arduino reads only the returned string through a http library (i forgot the exact name). It skips the return headers thanks to the library.
UPDATE: the library is called amcewen httpclient, youll find it on github. I have slightly modified it to fit for WiFi shield rather than the Ethernet shield.
I convert the returndata to variables and make my arduino set its pins based on the server calculations.
So it is possible to use a http client with a normal wifi hotspot phone to communicate to and from an Arduino.