use Drupal\Core\Database\Database;
$connection_info = Database::getConnectionInfo('default');

$db = $connection_info['default']['database'];
$u = $connection_info['default']['username'];
$p = $connection_info['default']['password'];
$current_date = date('Y-m-d_H-i-s');

$command = "mysqldump --user=$u --password=$p $db";
$dump = shell_exec($command);

header('Content-Description: File Transfer');
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename='.basename($db . '_' . $current_date . '.sql'));

echo $dump;
exit();