Error message

• Warning: Cannot modify header information - headers already sent by (output started at /home/greening/public_html/greenrights.com/includes/common.inc:2700) in drupal_send_headers() (line 1216 of /home/greening/public_html/greenrights.com/includes/bootstrap.inc).
• Error: Class 'DOMDocument' not found in filter_dom_load() (line 1098 of /home/greening/public_html/greenrights.com/modules/filter/filter.module).