Getting PEAR to work

I have a PHP script that requires PEAR, namely HTTP_Request

However, I get this error

Fatal error: Cannot instantiate non-existent class: http_request in /home/.parisian/site/directory/script.php on line 20

Other error

Warning: main(HTTP/Request.php): failed to open stream: No such file or directory in /home/.parisian/site/ on line 17

Fatal error: main(): Failed opening required ‘HTTP/Request.php’ (include_path=’.:/usr/local/lib/php’) in /home/.parisian/site/ on line 17