How do I invoke web services?
You should have a look at the Services module. You can use XMLRPC and/or REST server (and many more) to communicate with your remote server. Link to the documentation.


  A standardized solution of integrating external applications with Drupal. Service callbacks may be used with multiple interfaces like REST, XMLRPC, JSON, JSON-RPC, SOAP, AMF, etc. This allows a Drupal site to provide web services via multiple interfaces while using the same callback code.