Fix an issue with some PHP environments (mostly related to FastCGI or php-fpm,...

Fix an issue with some PHP environments (mostly related to FastCGI or php-fpm, common with nginx) where $_SERVER['SCRIPT_NAME'] is already populated with $_SERVER['PATH_INFO'] appended to it. In those cases, we should not blindly append PATH_INFO to SCRIPT_NAME, but check the latter first. This hopefully resolves #5 and closes #391.