I don't really know anything, but my guess is that you need to talk to someone in this office: http://www.phila.gov/property/facilities.html
A couple other possibilities: http://www.phila.gov/property/virtualcityhall/visitorsinfo.asp
They might at least know who to talk to.
You might also check with this office: http://www.phila.gov/mdo/Permits.html

Good luck, I think a wedding at City Hall would be beautiful and a lot of fun! Best wishes!

I think the only wedding you could have at City Hall is a justice of the peace wedding. Good luck!