Although it may not be possible to support every Sharepoint feature, I think it should be possible to support the basics. However, even the basics will take a bit of work and include working with the PHP WebDAV library, IE-only ActiveX controls, and possibly changing the user registry.
This site contains some information on how you need to configure WebDAV so that Office applications can open a file through WebDAV in read / write mode. Also read OpenDocuments Control and the registry key that it uses.
Using this information, you can probably create an Office-compatible WebDAV server using a PHP WebDAV library such as SabreDAV . SabreDAV's website mentions Open Office compatibility, but not Office compatibility, so you might have some kind of fix.
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