<?php
class SqlHelper
{
    private static $_instance;
    public $_dbname;
    private function __construct()
    {
    }
    //getInstance()方法必须设置为公有的,必须调用此方法
    public static function getInstance()
    {
        //对象方法不能访问普通的对象属性，所以$_instance需要设为静态的
        if (self::$_instance === null) {
            //                self::$_instance=new SqlHelper();//方式一    
            self::$_instance = new self(); //方式二        
        }
        return self::$_instance;
    }
    public function getDbName()
    {
        echo $this->_dbname;
    }
    public function setDbName($dbname)
    {
        $this->_dbname = $dbname;
    }
}
//    $sqlHelper=new SqlHelper();//打印：Fatal error: Call to private SqlHelper::__construct() from invalid context 
$A = SqlHelper::getInstance();
$A->setDbName('数据库名');
$A->getDbName();
//    unset($A);//移除引用
$B = SqlHelper::getInstance();
$B->getDbName();
$C = SqlHelper::getInstance();
$C->getDbName();

?>