PHP define()似乎无法处理include()

<?php
define("DB_HOST","localhost");
define("DB_USER","root");
define("DB_PASS","password");
define("DB_NAME","database");
?>

然后我在索引文件中这样使用它们:

include('definitions.php');
include('class_lib.php');

$testing = new databaseServer();

$testing->connect(DB_HOST,DB_USER,DB_PASS,DB_NAME);

我在databaseServer类中使用的函数是:

    function connect($host,$user,$pw,$db) {
        $this->con = mysql_connect($host,$user,$pw);
        if (!$this->con) {
            die('Could not connect: ' . mysql_error());
            }
        $this->selectDb($db);
        }

    function selectDb($database) {
        $this->db = mysql_select_db($database,$this->con);
        if (!$this->db) {
            echo "Could not Select database: " . mysql_error();
            }
        }

你知道为什么这样不行吗?我也尝试过将definitions文件放入class_lib文件的include中,但仍然不起作用。