how to fix PHP Fatal error: Class ‘DOMDocument’ not found

from the tail -f /var/log/httpd/error
[Tue Mar 10 16:01:30 2009] [error] [client 127.0.0.1] PHP Fatal error: Class 'DOMDocument' not found in /home/namran/src/....php on line 15, referer: http://...

the solution turn out to be
yum install php-xml