Get Last ID

PHP MySQL Get Last ID: Using PDO::lastInsertId() Safely

After inserting a row into a MySQL table with an AUTO_INCREMENT primary key, $pdo->lastInsertId() returns the newly generated ID as a string. You commonly need that ID immediately to insert child rows—such as saving order_items linked to a newly created orders row.

Fetching the ID Inside a Multi-Table Transaction

lastInsertId() is scoped strictly to your current database connection, so concurrent inserts from other visitors on different connections will never mix up your ID. Wrap parent and child inserts inside $pdo->beginTransaction() and $pdo->commit() so both succeed or roll back together.

<?php
declare(strict_types=1);

function createOrderWithItems(PDO $pdo, int $userId, array $items): int
{
    $pdo->beginTransaction();

    try {
        $total = array_sum(array_map(fn (array $item): float => $item['qty'] * $item['price'], $items));

        $orderStmt = $pdo->prepare(
            'INSERT INTO orders (user_id, total_amount) VALUES (:user_id, :total_amount)'
        );
        $orderStmt->execute([
            ':user_id' => $userId,
            ':total_amount' => round($total, 2),
        ]);

        // Cast the returned numeric string to int immediately
        $orderId = (int) $pdo->lastInsertId();

        $itemStmt = $pdo->prepare(
            'INSERT INTO order_items (order_id, sku, quantity, unit_price) VALUES (:order_id, :sku, :qty, :price)'
        );
        foreach ($items as $item) {
            $itemStmt->execute([
                ':order_id' => $orderId,
                ':sku' => $item['sku'],
                ':qty' => $item['qty'],
                ':price' => $item['price'],
            ]);
        }

        $pdo->commit();
        return $orderId;
    } catch (Throwable $e) {
        if ($pdo->inTransaction()) {
            $pdo->rollBack();
        }
        throw $e;
    }
}

Important Behavior on Multi-Row Inserts and UUIDs

If you insert multiple rows in a single INSERT INTO ... VALUES (...), (...) query, MySQL's lastInsertId() returns the ID of the first row in that batch, not the last row. Also, lastInsertId() returns '0' if the table has no AUTO_INCREMENT column (such as UUID primary keys).

Common Mistakes to Avoid

• Calling lastInsertId() after commit(): Wait until before commit()? Actually in PDO MySQL, calling lastInsertId() right after $stmt->execute() inside the transaction is safest—some drivers reset the value after commit().
• Running another INSERT before reading lastInsertId(): Each INSERT overwrites the connection's last insert ID. Store it in $orderId immediately after the parent insert.
• Returning raw lastInsertId() without (int) cast: PDO::lastInsertId() returns a string (to support 64-bit IDs on 32-bit PHP), so cast (int) when your method declares : int.

Frequently Asked Questions

Is PDO::lastInsertId() safe under high concurrent traffic?

Yes. MySQL tracks LAST_INSERT_ID() per TCP connection session, so User A's connection never sees User B's inserted ID.

What does lastInsertId() return if an INSERT IGNORE skipped a duplicate row?

It returns '0' because no new auto-increment row was created.

Does INSERT ... ON DUPLICATE KEY UPDATE update lastInsertId()?

Only if a new row was inserted (or if you explicitly pass id = LAST_INSERT_ID(id) in the update clause).