इसे आजमाएं
$db_num = $_POST['db_num'];
$db_amnt = $_POST['db_amnt'];
$items = array_combine($db_num,$db_amnt);
$pairs = array();
foreach($items as $key=>$value){
$pairs[] = '('.intval($key).','.intval($value).')';
}
mysql_query('INSERT INTO ' .$tablename_db. '(db_num, db_amnt) VALUES '.implode(',',$pairs));