<?php
function log_audit_trail(PDO $pdo, string $action, array $details, string $user = 'system') {
    try {
        $stmt = $pdo->prepare("INSERT INTO audit_log (user, action, details) VALUES (?, ?, ?)");
        $stmt->execute([$user, $action, json_encode($details)]);
    } catch (PDOException $e) {
        // For now, we will log the error to the system's error log.
        // In a production environment, this should be handled more robustly.
        error_log('Failed to log audit trail: ' . $e->getMessage());
    }
}