PHPでシングルトンモードを実装する方法

class Singleton
{
    //创建静态私有的变量保存该类对象
    static private $instance;
    //防止使用new直接创建对象
    private function __construct(){}
    //防止使用clone克隆对象
    private function __clone(){}
    static public function getInstance()
    {
        //判断$instance是否是Singleton的对象，不是则创建
        if (!self::$instance instanceof self) {
            self::$instance = new self();
        }
        return self::$instance;
    }
    public function test()
    {
        echo "我是一个单例模式";
    }
}
$sing = Singleton::getInstance();
$sing->test();
$sing2 = new Singleton(); //Fatal error: Uncaught Error: Call to private Singleton::__construct() from invalid context in
$sing3 = clone $sing; //Fatal error: Uncaught Error: Call to private Singleton::__clone() from context

class A
{
    public function show()
    {
        echo "我是A类函数，要调用B类函数和C类函数
";
        $b = new B();
        $b->show();
        $c = new C();
        $c->show('A');
    }
}
class B
{
    public function show()
    {
        echo "我是B类函数，要调用C类函数
";
        $c = new C();
        $c->show('B');
    }
}
class C
{
    public function show($parm)
    {
        echo "我是C类函数，现在被{$parm}类函数调用
";
    }
}
$a = new A();
$a->show();

class A
{
    public function show()
    {
        echo "我是A类函数，要调用B类函数和C类函数
";
        $b = Unit::getInstance('B');
        $b->show();
        $c = Unit::getInstance('C');
        $c->show('A');
    }
}
class B
{
    public function show()
    {
        echo "我是B类函数，要调用C类函数
";
        $c = Unit::getInstance('C');
        $c->show('B');
    }
}
class C
{
    public function __construct()
    {
        echo "C类对象被创建
";
    }
    public function show($parm)
    {
        echo "我是C类函数，现在被{$parm}类函数调用
";
    }
}
class Unit
{
    static public function getInstance($class)
    {
        static $arr = null;
        if (!isset($arr[$class]) || !$arr[$class] instanceof $class) {
            $arr[$class] = new $class();
        }
        return $arr[$class];
    }
}
$a = new A();
$a->show();