Error message

• Warning: Cannot modify header information - headers already sent by (output started at /var/www/html/includes/common.inc:2820) in drupal_send_headers() (line 1501 of /var/www/html/includes/bootstrap.inc).
• Error: Class 'SimpleXMLElement' not found in update_parse_xml() (line 390 of /var/www/html/modules/update/update.fetch.inc).