No, there is no need to symlink. Pear is a "packaging system" on it's own, so you need to "pear install" stuff after you have installed the pear package with aptitude.
sudo pear install mail
This downloads the mail stuff in /usr/share/php. And PHP has its own include_path which is similar to $PATH in the shell. It looks in many directories when you include/require files. By default, the include_path is ".:/usr/share/php:/usr/share/pear", so this should work: