COnverting PHP from Linux to Windows is relatively easy mostly involving the paths on the server containing the files.
WIndows using \
Linux uses /
If you're writing applications with cross platform compatability in mind, you shouldn't be hard coding filesystem separators. Use DIRECTORY_SEPARATOR instead.
All that said and done, there's much more to porting applications than simply replacing filesystem separators. It's unusual to find Windows boxes running sendmail/postfix, proftpd and/or BIND for example, so you'll have to rewrite that functionality for Windows (even if it's behind an abstraction layer, you've still got to do the work). Even Apache on Win32 in a production environment isn't all that common.
If you're restricting yourself to only supporting the basics that are fairly well supported on both platforms (Apache, MySQL), you'll likely be fine but you're seriously restricting the number of people who will find your program interesting.
Some of this might be common sense, but hey - people should know by now that I like to rant.