XML : Connect to local IP from outside via php

I have:

1. Data to connect to external server (IP, login, password).
2. Ready VPN-tunnel from external server to other machine.
3. Local IP (192.168.....?wsdl) of this machine with the pack of services to ask (SOAP-query).

Task: Send SOAP-request to Local IP and take an XML-answer.

I know how to realise SOAP-request, but I don't understand how to connect to local IP across VPN, using PHP.

For example, here i send XMLHttpRequest() to 'url' variable. I have direct access to 'url' link. But when the access behind the VPN...

  var url = "http://domain.com/MCAExportService/.....asmx";    function getKBM(client_iin) {  ...  }    var xmlHTTP = new XMLHttpRequest();      xmlHTTP.open("POST", url, true);  ...      xmlHTTP.send(xml_request);