I have two Apache servers running PHP. One accepts forward-slashes in the query string and passes it along to PHP in the expected way, for example:
http://server
You dont specify what PHP does with this url. Does it redirect to this page or try to read it?
There is probably some mod_rewrite rule to remove double slashes, or for some other purpose, which tries to redirect this to somewhere it should not.
Maybe a regex without ^ before http://