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Transfer funds between two accounts atomically with `PDO`

Implement Ledger::transfer. It must debit one account and credit another so that both rows change or neither does, and it must not hide a failure — the caller has to learn that the transfer did not happen.

Requirements: use the injected PDO handle, bind every value as a parameter, and leave no transaction open on any path.

<?php
class Ledger {
    public function __construct(private PDO $pdo) {}

    public function transfer(int $fromId, int $toId, int $amountCents): void {
        // your code here
    }
}

Write the implementation.

Wrap both UPDATEs in beginTransaction() … commit(), and in a catch call rollBack() and rethrow so the caller learns the transfer failed. With PDO::ERRMODE_EXCEPTION a failing statement throws by itself. Bind $fromId, $toId and the amount as parameters, never interpolate them.

✗Skipping the transaction and 'fixing' a failed debit with a compensating write
✗Catching the exception and returning, so the caller believes the transfer succeeded
✗Interpolating the ids and the amount into the SQL instead of binding them

→What does PDO::inTransaction() protect you from when transfer() may be called from an outer transaction?
→How would you make the transfer refuse to overdraw the source account inside the same transaction?

The transaction must be closed on every path: beginTransaction() opens it, commit() closes it on success, and the catch rolls back and rethrows — if it swallows the exception the caller will believe the transfer went through.

<?php
class Ledger {
    public function __construct(private PDO $pdo) {}

    public function transfer(int $fromId, int $toId, int $amountCents): void {
        $this->pdo->beginTransaction();
        try {
            $debit = $this->pdo->prepare(
                'UPDATE accounts SET balance = balance - :amt WHERE id = :id AND balance >= :amt'
            );
            $debit->execute(['amt' => $amountCents, 'id' => $fromId]);
            if ($debit->rowCount() !== 1) {
                throw new RuntimeException('Insufficient funds');   // ❌ rolls back
            }

            $credit = $this->pdo->prepare(
                'UPDATE accounts SET balance = balance + :amt WHERE id = :id'
            );
            $credit->execute(['amt' => $amountCents, 'id' => $toId]);

            $this->pdo->commit();                                    // ✅ both rows
        } catch (Throwable $e) {
            $this->pdo->rollBack();
            throw $e;                                                // ⚠️ never swallow
        }
    }
}

Three details an interviewer looks for:

statement throws on its own, so no manual return-value checks are needed.

makes the balance check part of the write itself, and 0 affected rows means "not enough funds".

straight injection hole; PDO does not escape anything "by itself".

PDO::ERRMODE_EXCEPTION (set when the PDO is constructed) — a failing
rowCount() after a conditional UPDATE — balance >= :amt in the WHERE
Bound parameters only. Interpolating $fromId into the query string is a

Open topic in roadmap →

Transfer funds between two accounts atomically with PDO

Requirements: use the injected PDO handle, bind every value as a parameter, and leave no transaction open on any path.

<?php
class Ledger {
    public function __construct(private PDO $pdo) {}

    public function transfer(int $fromId, int $toId, int $amountCents): void {
        // your code here
    }
}

Write the implementation.

<?php class Ledger { public function __construct(private PDO $pdo) {} public function transfer(int $fromId, int $toId, int $amountCents): void { $this->pdo->beginTransaction(); try { $debit = $this->pdo->prepare( 'UPDATE accounts SET balance = balance - :amt WHERE id = :id AND balance >= :amt' ); $debit->execute(['amt' => $amountCents, 'id' => $fromId]); if ($debit->rowCount() !== 1) { throw new RuntimeException('Insufficient funds'); // ❌ rolls back } $credit = $this->pdo->prepare( 'UPDATE accounts SET balance = balance + :amt WHERE id = :id' ); $credit->execute(['amt' => $amountCents, 'id' => $toId]); $this->pdo->commit(); // ✅ both rows } catch (Throwable $e) { $this->pdo->rollBack(); throw $e; // ⚠️ never swallow } } }

Transfer funds between two accounts atomically with PDO

Transfer funds between two accounts atomically with PDO

Transfer funds between two accounts atomically with `PDO`

Transfer funds between two accounts atomically with `PDO`