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.
the full classname (like
throws PEAR_Error on any problem
This function can not be called statically.