Hmm, well maybe it might be.
IE seems to handle spaces in the URL quite well.
The space in "hello world" is often handled in IE, but I have noticed that NN and a few others consider a space to be the end of the URL and do not include anything after that.
I'm sure this is true for other characters as well.
Try to use urlencode before setting a variable. Than should replace all your spaces with %20 which should be ok for ALL browsers.
Alternatively, you could use rawurlencode. Both are in the php documentation.