PSR-3: Logger Interface

Also Known As

TL;DR

Explanation

PSR-3 defines a standard LoggerInterface that any logging library can implement, allowing application code to type-hint LoggerInterface rather than a specific library. Methods map to RFC 5424 severity levels: emergency, alert, critical, error, warning, notice, info, debug. Each accepts a message string and optional context array (interpolated as {key} placeholders). Monolog is the most widely used PSR-3 implementation — it supports structured logging, multiple handlers (file, Slack, Datadog, Sentry), and formatters. Libraries and frameworks that accept a LoggerInterface become logger-agnostic.

Common Misconception

Why It Matters

Common Mistakes

• Type-hinting against a concrete logger class instead of LoggerInterface — locks the codebase to one library.
• Not using the context array parameter — embedding variables in the message string prevents structured log parsing.
• Using the wrong log level — debug for production-visible events, or error for expected business conditions.
• Not injecting the logger — using a static logger facade prevents swapping for a NullLogger in tests.

Code Examples

// Concrete dependency — cannot swap logger:
public function __construct(private Monolog\Logger $logger) {}

// Not using context array — unstructured log:
$this->logger->info('User ' . $userId . ' logged in from ' . $ip);

// Correct:
public function __construct(private Psr\Log\LoggerInterface $logger) {}
$this->logger->info('User logged in', ['user_id' => $userId, 'ip' => $ip]);

// PSR-3 Logger — always inject the interface, not a concrete logger
use Psr\Log\LoggerInterface;

class OrderService {
    public function __construct(private LoggerInterface \$logger) {}

    public function place(Cart \$cart): Order {
        \$this->logger->info('Placing order', ['cart_id' => \$cart->id]);
        try {
            \$order = \$this->createOrder(\$cart);
            \$this->logger->info('Order placed', ['order_id' => \$order->id]);
            return \$order;
        } catch (\Throwable \$e) {
            \$this->logger->error('Order failed', [
                'cart_id'   => \$cart->id,
                'exception' => \$e->getMessage(),
            ]);
            throw \$e;
        }
    }
}
// Levels: emergency, alert, critical, error, warning, notice, info, debug

References

• ↗ https://www.php-fig.org/psr/psr-3/
• ↗ https://github.com/Seldaek/monolog

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