I think for multiple check boxes insert, you have to coding like this in your php file

$c=$_REQUEST['chkbx'];

for($i=0;$i<count($c);$i++)
{
$val=$c[$i];
$sql="INSERT INTO eventdesc(show) VALUES($val);
$res=mysql_query($sql);
}

By this coding,you can insert all check boxes values in your DB which are checked.