KISS 原則 (シンプルに、バカにしてください)

// Complex (Bad)
class UserRepository {
    public function getUserWithDetails($userId) {
        return array_reduce(
            $this->db->query("SELECT * FROM users WHERE id = ?", [$userId])->fetch(),
            function($carry, $userData) {
                return array_merge(
                    $carry,
                    $this->enrichUserData(
                        $userData,
                        $this->getAdditionalData($userData['id'])
                    )
                );
            },
            []
        );
    }

    private function enrichUserData($userData, $additionalData) {
        return array_map(
            function($data) use ($userData) {
                return $this->transformData($data, $userData);
            },
            $additionalData
        );
    }
}

// Simple (Good)
class UserRepository {
    public function getUserWithDetails($userId) {
        $user = $this->db->query("SELECT * FROM users WHERE id = ?", [$userId])->fetch();
        if (!$user) {
            return null;
        }

        $user['address'] = $this->getAddress($userId);
        $user['orders'] = $this->getRecentOrders($userId);

        return $user;
    }

    private function getAddress($userId) {
        return $this->db->query("SELECT * FROM addresses WHERE user_id = ?", [$userId])->fetch();
    }

    private function getRecentOrders($userId) {
        return $this->db->query(
            "SELECT * FROM orders WHERE user_id = ? ORDER BY created_at DESC LIMIT 5",
            [$userId]
        )->fetchAll();
    }
}

// Complex (Bad)
class ConfigManager {
    private $config = [];

    public function get($key) {
        return array_reduce(
            explode('.', $key),
            function($carry, $segment) {
                return is_array($carry) && isset($carry[$segment]) 
                    ? $carry[$segment] 
                    : null;
            },
            $this->config
        );
    }

    public function set($key, $value) {
        $segments = explode('.', $key);
        $current = &$this->config;

        while (count($segments) > 1) {
            $segment = array_shift($segments);
            if (!isset($current[$segment]) || !is_array($current[$segment])) {
                $current[$segment] = [];
            }
            $current = &$current[$segment];
        }

        $current[array_shift($segments)] = $value;
    }
}

// Simple (Good)
class ConfigManager {
    private $config = [];

    public function get($key) {
        if (isset($this->config[$key])) {
            return $this->config[$key];
        }
        return null;
    }

    public function set($key, $value) {
        $this->config[$key] = $value;
    }
}

// Complex (Bad)
function getNextBusinessDay($date) {
    $timestamp = strtotime($date);
    $daysToAdd = 1;

    while (true) {
        $nextDay = strtotime("+$daysToAdd days", $timestamp);
        $dayOfWeek = date('N', $nextDay);

        if ($dayOfWeek < 6) {
            $holidays = $this->getHolidays();
            $dateString = date('Y-m-d', $nextDay);

            if (!in_array($dateString, $holidays)) {
                return $dateString;
            }
        }
        $daysToAdd++;
    }
}

// Simple (Good)
function getNextBusinessDay($date) {
    $nextDay = new DateTime($date);
    $nextDay->modify('+1 day');

    while ($this->isWeekendOrHoliday($nextDay)) {
        $nextDay->modify('+1 day');
    }

    return $nextDay->format('Y-m-d');
}

private function isWeekendOrHoliday(DateTime $date) {
    $dayOfWeek = $date->format('N');
    $isWeekend = ($dayOfWeek >= 6);
    $isHoliday = in_array($date->format('Y-m-d'), $this->holidays);

    return $isWeekend || $isHoliday;
}

// Complex (Bad)
function validateUserInput($data) {
    return (
        isset($data['email']) && 
        filter_var($data['email'], FILTER_VALIDATE_EMAIL) &&
        isset($data['password']) && 
        strlen($data['password']) >= 8 &&
        preg_match('/[A-Z]/', $data['password']) &&
        preg_match('/[a-z]/', $data['password']) &&
        preg_match('/[0-9]/', $data['password']) &&
        (!isset($data['phone']) || 
            (preg_match('/^\+?[1-9]\d{1,14}$/', $data['phone']))
        )
    ) ? true : false;
}

// Simple (Good)
class UserValidator {
    public function validate($data) {
        $errors = [];

        if (!$this->isValidEmail($data['email'])) {
            $errors['email'] = 'Invalid email address';
        }

        if (!$this->isValidPassword($data['password'])) {
            $errors['password'] = 'Password must be at least 8 characters with mixed case and numbers';
        }

        if (isset($data['phone']) && !$this->isValidPhone($data['phone'])) {
            $errors['phone'] = 'Invalid phone number';
        }

        return empty($errors) ? true : $errors;
    }

    private function isValidEmail($email) {
        return filter_var($email, FILTER_VALIDATE_EMAIL) !== false;
    }

    private function isValidPassword($password) {
        if (strlen($password) < 8) {
            return false;
        }

        return preg_match('/[A-Z]/', $password) 
            && preg_match('/[a-z]/', $password) 
            && preg_match('/[0-9]/', $password);
    }

    private function isValidPhone($phone) {
        return preg_match('/^\+?[1-9]\d{1,14}$/', $phone);
    }
}