Mysqli多个查询

Question

嗨，大家好我的SQL查询有问题。我想要做的是第一个查询应该插入，然后插入行我想选择它，并在第三个查询中使用插入到另一个表。

1) $sql .= "update player set money='".$total."' WHERE username = '" . $_SESSION['username'] . "';";

2) $uname = $_SESSION['username'];
$sql .= "INSERT INTO transaction(username, transactionType, postWinning, timeOfTransaction) VALUES('$uname', 'Buy', '$result', '$date')";

3) $sql = "Select transactionID from transaction order by transactionID DESC LIMIT 1";

4) $sql .= "INSERT INTO buySellDetail(transactionID, stockId, quantity, price) VALUES('$lastID', '$code', '$share', '$total');";

atm第3和第4个查询在我的代码中被注释掉，因为它不起作用。我怎么想写第三个查询？谢谢