Debian doesn't have nativ package support for php eaccelerator. Which is used by moodle for example.

echo 'deb-src http://debian.mcmillan.net.nz/debian lenny awm' >> /etc/apt/sources.list
apt-get --build source php5-eaccelerator
dpkg -i php5-eaccelerator_0.9.5.3-0+lenny1_amd64.deb