Log Levels & When to Use Each

TL;DR

Explanation

Standard levels (RFC 5424 / PSR-3): EMERGENCY (system unusable), ALERT (immediate action needed), CRITICAL (component failure), ERROR (operation failed, handled), WARNING (unexpected but recovered), NOTICE (normal but significant), INFO (business events, milestones), DEBUG (developer details, queries, input). Production: log INFO and above. Development: log DEBUG and above. Rules: ERROR = operation that failed and was caught (payment declined). WARNING = degraded behaviour (cache miss, rate limited). INFO = key business events (order placed, user registered). DEBUG = everything else. CRITICAL = component down (DB, cache unavailable).

Common Misconception

Why It Matters

Common Mistakes

• Logging expected user errors as ERROR — 401 Unauthorized is INFO, not ERROR.
• Not logging at INFO for key business events — hard to audit.
• DEBUG logs in production — performance and cost impact.
• CRITICAL for recoverable errors — saves CRITICAL for actual outages.

Code Examples

Log::error('User login failed'); // Expected behaviour — should be INFO/WARNING
Log::debug('Processing payment', $data); // In production — noise

Log::info('User login failed', ['user_email' => $email, 'reason' => 'wrong_password']);
Log::error('Payment gateway timeout', ['order_id' => $orderId, 'gateway' => 'stripe']);
Log::critical('Database connection failed', ['host' => $dbHost]);

// In config — production:
// monolog.level: info (suppresses debug)

References

• ↗ https://www.php-fig.org/psr/psr-3/

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