https://bugs.launchpad.net/ubuntu/+source/phpunit/+bug/701544
and specially this comment for ubuntu 11.04 and may be for some others too
even though i did ths steps as recommended above, i didn't get it to work but after i upgraded pear from 1.9.1 to 1.9.2 it's working perfectly just do this
speshu@speshu-laptop:~$ sudo pear upgrade pear
downloading PEAR-1.9.2.tgz ...
Starting to download PEAR-1.9.2.tgz (295,120 bytes)
.....................................................done: 295,120 bytes
upgrade ok: channel://pear.php.net/PEAR-1.9.2
PEAR: Optional feature webinstaller available (PEAR's web-based installer)
PEAR: Optional feature gtkinstaller available (PEAR's PHP-GTK-based installer)
PEAR: Optional feature gtk2installer available (PEAR's PHP-GTK2-based installer)
PEAR: To install optional features use "pear install pear/PEAR#featurename"