There is bug that I've come across when running ISPConfig 2.2.35 where getting PHP files via WebDAV pulls down the executed script vs. the actual PHP content.
To fix this problem you'll want to add directives via ISPConfig's Apache Directives (Optional) setting. I add the following that prevents PHP scripts from executing when downloading via WebDAV
# Fix for PHP WebDAV
php_value engine off
WebDAV is an excellent alternative to FTP because it can be secure (with optional SSL), HTTP(S) is optimized on most routers and networks, and its fast as hell. Its also supported on various OS's natively from OS X to Windows, and in 'professional' web IDEs like Dreamweaver (for designers) and phpED (for developers). With this fix I can develop faster using Dw and www.themedreamer.com
developing WordPress themes.