The web service could be a SOAP asmx or a WCF Service. Assumption here is that the IIS on Windows is serving the web-service and Apache on Linux with PHP 5.3 consuming it.
The hosting system should not matter, calling a web service is the same (in fact, that's one of the points of setting up a web service).
PHP has built in SOAP objects (Manual Entry for it). Those should be able to access it without any issue.