导出txt格式：
// Load the DB utility class
$this->load->dbutil();
// Backup your entire database and assign it to a variable
$backup=&$this->dbutil->backup(array('format'=>'txt'));
// Load the file helper and write the file to your server
$this->load->helper('file');
write_file('backup.sql',$backup);
gzip格式：
// Load the DB utility class
$this->load->dbutil();
// Backup your entire database and assign it to a variable
$backup=&$this->dbutil->backup();
// Load the file helper and write the file to your server
$this->load->helper('file');
write_file('mybackup.gz',$backup);
zip格式
// Load the DB utility class
$this->load->dbutil();
// Backup your entire database and assign it to a variable
$backup=&$this->dbutil->backup(array('format'=>'zip'));
// Load the file helper and write the file to your server
$this->load->helper('file');
write_file('backup.zip',$backup);
