Update PHP for Move to Windows

My client's webhost moved their website from UNIX to a Windows server without consulting them. As a result, the exisitng PHP programming no longer works with errors like: "Fatal error: Cannot redeclare class exception in F:[login to view URL] on line 9" The website has a restricted login area for authorised clients only as well as an admin area that allows the creation of directories, uploading files and allowing access privlidges to directories. When authorised clients log-in, they see links to the directories they are authorised to access. It has beeen 2.5 years since the website was developed and I no longer have access to the original programmers. I need someone who has experience in rectifying this sort of problem and can do it on a pretty short turn around. May lead to further development work as this client is due (overdue!) for an update!!