Xampp ships with PHP, and apparently PEAR and some PEAR packages installed.
I think the version of pear packages that is chosen to be packaged, should be at the very least recent enough to be COMPATIBLE with the packaged version of PHP.
I've found out the version of the Mail pear package is dated 2005, and it is unusable with the version of PHP included in Xampp. Trying to call Mail::factory() will cause the error:
Assigning the return value of new by reference is deprecated
55 $mailer = &new $class($params);