It's not impossible, but it's not desirable.
In any case, URL structures like this are not recommended.
Indeed. I used to be an administrator for a wiki with this URL setup. I assure you there have been way too many cases where the existence of files (non-PHP files, to be exact - such as FAQ, LICENSE and whatever) and directories broke many articles. For this reason, I strongly object using the root path for your articles. However, if the reason is to un-break old links - we can definitely add 301 Permanent Redirects for /$1 to /wiki/$1 (unless $1 isa file on our server) so clients and spiders/robots will be aware of the new, correct URL path.