Description
   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.