mysqli_connect（）：php_network_getaddresses：getaddrinfo失败：没有这样的主机是已知的

Question

当我尝试在本地服务器上运行它时，我收到此错误：

mysqli_connect(): php_network_getaddresses: getaddrinfo failed: No such host is known

任何人都可以帮助我吗？谢谢。

connection.php

<?php
require("constants.php");
//1:create connection
$connection = mysqli_connect("DB_SERVER","DB_USER","DB_PASS");
    if (!$connection){
    die("Database connection failed:" . mysql_error());
    }
//2:Select a database to use
$db_select= mysqli_select_db(DB_NAME, "widget_corp");
    if (!$db_select){
        die("Database selection failed:" . mysql_error());
    }
?>

constants.php

<?php 

//Database constants
define("DB_SERVER", "localhost");
define("DB_USER", "root");
define("DB_PASS", "helloworld");
define("DB_NAME", "widget_corp");
?>