Warning: require_once(PEAR.php) [function.require-once]: failed to open stream: No such file or directory in /usr/local/php4/lib/php/Mail.php on line 21

Fatal error: require_once() [function.require]: Failed opening required 'PEAR.php' (include_path='.:/usr/local/lib/php') in /usr/local/php4/lib/php/Mail.php on line 21