大问题,php不认mysql接口函授,怎么办???

网页提示Fatal error: Call to undefined function: mysql_connect() in /usr/src/www/inc/conn.php on line 5
下边是conn.php的内容:
<?php
$host="localhost";
$username="root";
$pwd="******";
mysql_connect($host,$username,$pwd) or die("Can not connect to database");
?>
怎么办,到底是怎么回事?我用的是rh9自带的mysql.
(我是初学者,大家可别笑噢)