工作实例
要了解这个在实践中是怎么工作的，让我们回到前面讨论过的银行转帐的例子.我们假设你的任务是建立一个简单的Web应用程序，让用户在他们的银行帐户间转帐。我们再进一步假设一个单独用户的帐户存储在一个MySQL数据库中，如下所示：
mysql> SELECT * FROM accounts;
+----+------------+---------+
| id | label | balance |
+----+------------+---------+
| 1 | Savings #1 | 1000 |
| 2 | Current #1 | 2000 |
| 3 | Current #2 | 3000 |
+----+------------+---------+
3 rows in set (0.34 sec)
现在，需要建立一个简单的界面，使用户能够输入一个现金数额，实现从一个帐户到另一个的转帐。实际的“交易”将用两个UPDATE语句来执行，一个将转帐金额从源帐户取出，即借方，另一个将转帐金额记入目标帐户，即贷方。假设我们所做的是在帐户之间进行转帐，那么所有帐户的可用结余总额（00）应该一直保持不变。
列表B显示了可能的代码：
列表 B
<?php
// connect to database
$dbh = mysqli_connect("localhost", "user", "pass", "test") or die("Cannot connect");
// turn off auto-commit
mysqli_autocommit($dbh, FALSE);
// look for a transfer
if ($_POST['submit'] && is_numeric($_POST['amt'])) {
// add $$ to target account
$result = mysqli_query($dbh, "UPDATE accounts SET balance = balance + " . $_POST['amt'] . " WHERE id = " . $_POST['to']);
if ($result !== TRUE) {
mysqli_rollback($dbh); // if error, roll back transaction
}
// subtract $$ from source account
$result = mysqli_query($dbh, "UPDATE accounts SET balance = balance - " . $_POST['amt'] . " WHERE id = " . $_POST['from']);
if ($result !== TRUE) {
mysqli_rollback($dbh); // if error, roll back transaction
}
// assuming no errors, commit transaction
mysqli_commit($dbh);
}
// get account balances
// save in array, use to generate form
$result = mysqli_query($dbh, "SELECT * FROM accounts");
while ($row = mysqli_fetch_assoc($result)) {
$accounts[] = $row;
}
// close connection
mysqli_close($dbh);
?>
<html>
<head></head>
<body>
<h3>TRANSFER</h3>
<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post">
Transfer $ <input type="text" name="amt" size="5"> from
<select name="from">
<?php
foreach ($accounts as $a) {
echo "<option value="" . $a['id'] . "">" . $a['label'] . "</option>";
}
?>
</select>
to
<select name="to">
<?php
foreach ($accounts as $a) {
echo "<option value="" . $a['id'] . "">" . $a['label'] . "</option>";
}
?>
</select>
<input type="submit" name="submit" value="Transfer">
</form>
<h3>ACCOUNT BALANCES</h3>
<table border=1>
<?php
foreach ($accounts as $a) {
echo "<tr><td>" . $a['label'] . "</td><td>" . $a['balance'] . "</td></tr>";
}
?>
</table>
</body>
</html>
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