Error message

• Warning: Cannot modify header information - headers already sent by (output started at /var/www/html/includes/common.inc:2791) in drupal_send_headers() (line 1499 of /var/www/html/includes/bootstrap.inc).
• PDOException: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away: SELECT 1 AS expression FROM {variable} variable WHERE ( (name = :db_condition_placeholder_0) ); Array ( [:db_condition_placeholder_0] => cron_last ) in variable_set() (line 1258 of /var/www/html/includes/bootstrap.inc).