致命错误：在第6行的/home/ck/public_html/conn.php中调用未定义的函数mysqli_connect（）

Question

conn.php

<?php
$user_name="root";
$password="*****";
$database="user";
$server="localhost";
$link=mysqli_connect($server,$user_name,$password,$database);
if(!link)
{
    echo "not";
}
else
{
    echo "";
}

＆GT;

当我访问我的网站时，显示

Fatal error: Call to undefined function mysqli_connect() in /home/ck/public_html/conn.php on line 6

如何解决此问题？请帮忙

谢谢