Put a WiFi enabled Raspberry Pi in your backpack. Connect your audio interface of choice, and control with your phone.

1. Install Raspbian.
2. Install Apache and PHP.
3. Install Dnsmasq and Hostapd.
4. Place all files at /var/www/.
5. Make directory /var/www/html/data and make sure that Apache has write permissions.
6. Connect your phone to the WiFi network that you configured in step 3.

You should now be able to visit the configured IP Adress in your webbrowser.