单例模式:一个类仅允许被实例化一次

<?php 

class Hubby

{

private function __construct() {}

private function __clone() {}

protected static $instance = null;

public static function getInstance()

{

if (is_null(static::$instance)) {

static::$instance = new static();

}

return static::$instance;

}

}

$hubby1 = Hubby::getInstance();

$hubby2 = Hubby::getInstance();

echo ($hubby1 instanceof Hubby) ? '是' : '不是';

echo '<br>';

echo ($hubby2 instanceof Hubby) ? '是' : '不是';

echo '<br>';

echo ($hubby1 === $hubby2) ? '完全相等' : '不相等';

echo '<br>';

var_dump($hubby1, $hubby2);