Set the kind of frontend that should be retrieved from the singleton() method. If the class does not exist, the method changes all underscores (_) into directory separators (like PEAR_Frontend_CLI to PEAR/Frontend/CLI) and appends .php and then checks to see if the file can be included. If the class does exist after all of this, a simple check is made to see if the userConfirm() method exists, and then a new frontend object is returned. Any failure causes a PEAR_Error to be returned.