Depending on your operating system probably there should be a pear package.
For example on ubuntu linux it is php-pear
, and you can install it like this:
sudo apt-get install php-pear
If pear is only missing from your include_path, you should add it in your php.ini files. (somewhere around /etc/php*)
In my case /etc/php5/*/php.ini
:
include_path = ".:/usr/share/php"
But you should verify the path.