php-mysql包装类__destruct方法无法关闭数据库

有人能解释为什么从类析构函数调用mysql_close（）时失败吗 mysql_error（）报告"连接关闭失败。"

<?php
class Database
{
    private $link_id = 0;
    private $query_id = 0;
    public $error = "";
    public $errorNumber = 0;
    public $affectedRows = 0;
    public function __construct($server, $user, $pass, $database)
    {
        $this->link_id = @mysql_connect($server, $user, $pass, false);
        if (!$this->link_id)
        {
            $this->DisplayError("Could not connect to server: <b>$this->server</b>.");
            die(mysql_error());
        }
        if(!@mysql_select_db($database, $this->link_id))
        {
            $this->DisplayError("Could not open database: <b>$this->database</b>.");
            die(mysql_error());
        }
    }
    public function __destruct()
    {
        if ($this->link_id)
        {
            if(!@mysql_close($this->link_id))
            {
                $this->DisplayError("Connection close failed.");
                die(mysql_error());
            }
        }
    }