The best solution seems to be to get rid of the old, broken version, install the CGI version of PHP and then install the PEAR package manager that way. You can do that from the console. Follow the instructions, the defaults are okay.
dpkg --purge php4-pear apt-get install php4-cgi ( cd /usr/bin ; ln -s php4 php ) wget http://go-pear.org -O - | php
lib/main.php:944: Notice: PageInfo: Cannot find action page