PHP单例模式应用详解

class Database {
    // 静态私有变量保存唯一实例
    private static $instance = null;
// 私有构造函数
private function __construct() {
    // 连接数据库操作
    echo "数据库已连接\n";
}

// 禁止克隆
private function __clone() {}

// 获取实例的静态方法
public static function getInstance() {
    if (self::$instance === null) {
        self::$instance = new self();
    }
    return self::$instance;
}

// 示例方法
public function query($sql) {
    echo "执行SQL: $sql\n";
}
}
// 使用示例
$db1 = Database::getInstance();
$db2 = Database::getInstance();
var_dump($db1 === $db2); // 输出 true，说明是同一个实例
$db1->query("SELECT * FROM users");

class Config {
    private static $instance = null;
    private $data = [];
private function __construct() {
    $this->data = include 'config.php'; // 假设配置文件
}

private function __clone() {}

public static function getInstance() {
    if (self::$instance === null) {
        self::$instance = new self();
    }
    return self::$instance;
}

public function get($key) {
    return $this->data[$key] ?? null;
}
}

class Logger {
    private static $instance = null;
    private $file;
private function __construct() {
    $this->file = fopen('app.log', 'a');
}

public static function getInstance() {
    if (self::$instance === null) {
        self::$instance = new self();
    }
    return self::$instance;
}

private function __clone() {}

public function log($message) {
    $time = date('Y-m-d H:i:s');
    fwrite($this->file, "[$time] $message\n");
}

public function __destruct() {
    if ($this->file) {
        fclose($this->file);
    }
}
}

Logger::getInstance()->log("用户登录成功");