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What is PDO in PHP, and what advantages does it provide over older database extensions like mysqli for interacting with a database? What is a prepared statement in PDO, and why is using one considered essential for writing secure database queries? How do you use named placeholders in a PDO prepared statement, and how do they improve readability compared to positional question mark placeholders? How do you configure PDO's error handling mode to ensure database errors are properly reported as catchable exceptions rather than silently failing? How does connection pooling and persistent connections work with PDO, and what tradeoffs should be considered before enabling persistent connections? How should an application handle database connection failures gracefully, including implementing retry logic and connection health checks for a reliable production system?
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How do database transactions work in PDO, and why are they important for ensuring multiple related database operations either all succeed together or all fail together?

Intermediate

Answer

A transaction lets you group multiple database operations together so that they are treated as a single atomic unit, meaning either every operation within the transaction succeeds and is permanently committed, or if any operation fails, the entire transaction can be rolled back, undoing every change made so far, which is essential for operations like transferring money between two accounts, where you never want one account to be debited without the corresponding credit also succeeding.
$pdo->beginTransaction();
try {
    $pdo->exec('UPDATE accounts SET balance = balance - 100 WHERE id = 1');
    $pdo->exec('UPDATE accounts SET balance = balance + 100 WHERE id = 2');
    $pdo->commit();
} catch (Exception $e) {
    $pdo->rollBack();
}
Real-world example A banking application wraps a fund transfer between two accounts in a transaction, ensuring that if the second update somehow fails after the first one succeeds, the entire operation rolls back completely, guaranteeing the total money in the system always remains correctly balanced.

Common follow-ups: What happens to a transaction if the script crashes before either commit or rollBack is called?;What are database isolation levels and how do they relate to transactions?

Error & Exception Handling;Security
All 7 PDO & Databases questions