php: 单一模型

<?php
class Singleton
{
    private static $instace;
    
    function __construct()
    {
        if(!self::$instace) //create it
        {
            self::$instace = $this;
            echo 'New Instance!<br>';
            
        }
        else
        {
            echo 'Old Instance!<br>';
        }
        return self::$instace;
    }
}

$s1 = new Singleton();
$s2 = new Singleton();
$s3 = new Singleton();
?>