PHP Post Data Over VPN

VPNs are at a network layer below PHP, PHP won't know or care that the connection is over a VPN or a normal connection. It's handled by the network stack.

If you use a permanent one (e.g. IPSEC) then PHP doesn't need to create the connection, it's just there to use when PHP connects to an IP address that is in the VPN. It is selected to use by the network layer when it does the routing, not by PHP. This is true even if you create the VPN on demand, as jderda suggested using exec() or similar. But a permanent connection is better (IPSEC).

So to answer your questions:

1. The question doesn't make sense, the only way PHP could do this is using PPTP or similar and exec() to bring the connection up, but better to use IPSEC
2. If the VPN connection hangs/dies PHP won't get a connection to the remote end and will timeout the connection.
3. Yes it is.

From PHP point of view, the VPN is just a plain network connection. It does not require additional handling.

If you want to dynamicaly estabilish a VPN connection, you'll probably need to use exec() and some commandline tool for estabilishing a connection. But as such connection doesn't interfere with normal network communication (as long as it's properly configured, with other subnet ip range), you should estabilish it once and keep it active for PHP and other apps to use.