PHP 抽象设计模式：从理念到实战的进阶指南

class Singleton {
    // 存储唯一实例的静态变量
    private static $instance = null;
    
    // 私有构造方法，防止外部实例化
    private function __construct() {
        // 初始化代码
    }
    
    // 私有克隆方法，防止克隆
    private function __clone() {
        // 可以抛出异常或直接忽略
    }
    
    // 防止反序列化创建新实例
    private function __wakeup() {
        // 可以抛出异常
    }
    
    // 静态方法，提供全局访问点
    public static function getInstance() {
        if (self::$instance === null) {
            self::$instance = new self();
        }
        return self::$instance;
    }
    
    // 类的其他方法
    public function doSomething() {
        // 业务逻辑
    }
}

public static function getInstance() {
    if (self::$instance === null) {
        // 加锁防止多线程同时创建实例
        synchronized(self::class, function() {
            if (self::$instance === null) {
                self::$instance = new self();
            }
        });
    }
    return self::$instance;
}

private function __wakeup() {
    throw new Exception("Cannot unserialize singleton");
}

class Database {
    private static $instance = null;
    private $connection;
    
    // 数据库配置
    private $host = 'localhost';
    private $db   = 'my_database';
    private $user = 'username';
    private $pass = 'password';
    private $charset = 'utf8mb4';
    
    private function __construct() {
        $dsn = "mysql:host=$this->host;dbname=$this->db;charset=$this->charset";
        $options = [
            PDO::ATTR_ERRMODE            => PDO::ERRMODE_EXCEPTION,
            PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
            PDO::ATTR_EMULATE_PREPARES   => false,
        ];
        try {
            $this->connection = new PDO($dsn, $this->user, $this->pass, $options);
        } catch (\PDOException $e) {
            throw new \PDOException($e->getMessage(), (int)$e->getCode());
        }
    }
    
    private function __clone() {}
    private function __wakeup() {}
    
    public static function getInstance() {
        if (self::$instance === null) {
            self::$instance = new self();
        }
        return self::$instance;
    }
    
    // 获取数据库连接
    public function getConnection() {
        return $this->connection;
    }
    
    // 封装常用查询方法
    public function query($sql, $params = []) {
        $stmt = $this->connection->prepare($sql);
        $stmt->execute($params);
        return $stmt;
    }
}

// 使用方式
$db = Database::getInstance();
$users = $db->query("SELECT * FROM users WHERE status = ?", [1])->fetchAll();

class Config {
    private static $instance = null;
    private $config = [];
    
    private function __construct() {
        // 加载配置文件
        $this->loadConfig();
    }
    
    private function __clone() {}
    private function __wakeup() {}
    
    public static function getInstance() {
        if (self::$instance === null) {
            self::$instance = new self();
        }
        return self::$instance;
    }
    
    private function loadConfig() {
        // 加载主配置文件
        $mainConfig = require __DIR__ . '/config/main.php';
        // 加载环境配置文件（开发/生产）
        $env = getenv('APP_ENV') ?: 'development';
        $envConfig = require __DIR__ . "/config/{$env}.php";
        
        // 合并配置
        $this->config = array_merge($mainConfig, $envConfig);
    }
    
    // 获取配置项
    public function get($key, $default = null) {
        $keys = explode('.', $key);
        $value = $this->config;
        
        foreach ($keys as $k) {
            if (!isset($value[$k])) {
                return $default;
            }
            $value = $value[$k];
        }
        
        return $value;
    }
    
    // 设置配置项
    public function set($key, $value) {
        $keys = explode('.', $key);
        $config = &$this->config;
        
        foreach ($keys as $i => $k) {
            if ($i === count($keys) - 1) {
                $config[$k] = $value;
                break;
            }
            if (!isset($config[$k])) {
                $config[$k] = [];
            }
            $config = &$config[$k];
        }
        
        return $this;
    }
}

// 使用方式
$config = Config::getInstance();
$apiKey = $config->get('services.payment.api_key');
$timeout = $config->get('app.timeout', 30);

class Logger {
    private static $instance = null;
    private $logFile;
    
    private function __construct() {
        $this->logFile = __DIR__ . '/logs/app.log';
        // 确保日志目录存在
        $this->ensureLogDirectory();
    }
    
    private function __clone() {}
    private function __wakeup() {}
    
    public static function getInstance() {
        if (self::$instance === null) {
            self::$instance = new self();
        }
        return self::$instance;
    }
    
    private function ensureLogDirectory() {
        $dir = dirname($this->logFile);
        if (!is_dir($dir)) {
            mkdir($dir, 0755, true);
        }
    }
    
    public function log($message, $level = 'info') {
        $timestamp = date('Y-m-d H:i:s');
        $logLine = "[$timestamp] [$level] $message" . PHP_EOL;
        file_put_contents($this->logFile, $logLine, FILE_APPEND);
    }
    
    public function info($message) {
        $this->log($message, 'info');
    }
    
    public function error($message) {
        $this->log($message, 'error');
    }
    
    public function warning($message) {
        $this->log($message, 'warning');
    }
}

// 使用方式
$logger = Logger::getInstance();
$logger->info('User login successful');
$logger->error('Database connection failed');

public static function resetInstance() {
    // 仅在测试环境下启用
    if (getenv('APP_ENV') === 'testing') {
        self::$instance = null;
    }
}