Master server updates 15 times per second? Where did you get this figure from? Also, "Master server" is a Unity feature, not TNet.
TNet sends data when you tell it to send data via tno.Send. It keeps the connection alive by sending periodic pings, but that's it.
No, you won't be able to get TNet to talk to PHP. The two have nothing in common.
TNet doesn't have a master server. TNet has lobby server and game server features. Both can be active from the same executable, and can even be launched from within Unity without having to create any stand-alone executable. In either case, TNet has no database connectivity. If you need it, you will need to add it yourself by modifying the TNet's server component.