I have made a Xamarin app for Android and WP, which is basically a music player, with the ability to sync music between devices, similar to Spotify, but it's used to sync music that you have downloaded as an mp3/flac or whatever in any way. Probably Google Music can do the same, but that's not matter.
I'm running a PHP api on my Raspberry Pi 3 Model B (with Raspbian), to which these clients are making requests, e.g. to get the list of songs a user have uploaded, upload one, remove one etc..
Currently, the Raspberry serves as the main server of this whole system, so it stores the music files themselves. I'd like to make it somewhat "portable". Is it possible for the Raspberry to host a WiFi connection, without internet? Like the Raspberry creates a WiFi access point, mobiles can connect to it, and through Raspberry's IP address, be able to access that PHP API, and upload/download files to/from it?
I'm not good at networking, so the main problem could be that I don't really know how this is called. VPN maybe?