Changed DOCROOT to work with mod_vhost_alias

The mod_vhost_alias module in Apache 2.2 and below reports an unexpected DOCUMENT_ROOT to PHP. It's fixed in Apache 2.4 apparently (see https://issues.apache.org/bugzilla/show_bug.cgi?id=26052), but in the meantime this little string manip works instead of relying on $_SERVER['DOCUMENT_ROOT']. 

Only tested this on Ubuntu w/Apache 2.2