单例模式 Singleton

class A{
    private static $instance = null;
 
    private function __contruct(){}
 
    public static getInstance()
    {
    if (is_null(self::$instance)) {
            self::$instance = new static(); //此处注意和new self的区别
        }
        return self::$instance;
    }
}
 
调用:
$a = A::getInstance();
$a->getInstance();