class Test{
    private static $_instance;
    private function __construct(){
    }  
    
    public function __clone(){
        //防止被复制
        trigger_error("Clone is not allow!",E_USER_ERROR);
    }

    public static function getInstance(){
        if(!(self::$_instance instanceof self)){
           //*****************  
            echo "测试地方一";
           //*****************
            self::$_instance = new self;
        }  
       return self::$_instance;
    }

    public function test(){
        echo "ok";
    }
}

<?php

require_once(dirname(__FILE__).'/test.class.php');
$test = Test::getInstance();
$test->test();

$test2 = Test::getInstance();
$test2->test();

测试地方一okok