I had the same problem.
It is caused by not having the PEAR system in the PHP5 include path.
You need to edit the php.ini include path
You need to do this from the console.
My php.ini file lives at /etc/php5/apache/php.ini
In that file, look for 'Paths and Directories' (around line 490)
I added the PEAR directory so the line is as follows:
include_path="/usr/share/awl/inc:/usr/share/php"
In the php.ini, a colon ':' character separates each include path.
After saving the file, you need to restart apache. (sudo /etc/init.d/apache2 restart)
This solved my problem.