php - 如何限制单例类被克隆？

Question

我知道如何在 php 中创建单例类。但我很困惑，如果我们能够克隆它，那么它就浪费了该类。

我可以知道如何限制单例类的克隆吗？请检查以下代码:

class DatabaseConnection {

private static $singleton_instance = null;

private function construct__() {
    // Declaring the constructor as private ensures
    // that this object is not created as a new intance
    // outside of this class.  Therefore you must use the
    // global_instance function to create this object.
}

public static function global_instance() {
    static $singleton_instance = null;
    if($singleton_instance === null) {
        $singleton_instance = new DatabaseConnection();
    }

    return($singleton_instance);
}
}

Answer 1

只需将您的函数 __clone() 设为私有(private)即可。

private function __clone() { }

如果您尝试访问它，它将抛出 fatal error 。