如何在另一个页面中导入数据库连接

我是php的新手，我想将数据库连接细节从db.php导入到另一个页面，但它不起作用。

如果代码需要设置数据库连接，则每页

$bd=mysqli_connect("localhost","root","","dbgadget");

请帮助我或举一个例子，我如何访问其他页面中的数据库连接详细信息。

Db.php

<?php
$mysql_hostname = "localhost";
$mysql_user = "root";
$mysql_password = "";
$mysql_database = "dbgadget";

$bd = mysqli_connect($mysql_hostname, $mysql_user,   
$mysql_password,$mysql_database) ;

if (mysqli_connect_errno())
{
    echo "Failed to connect to database: " . mysqli_connect_error();
}
mysqli_close($bd);
?>

产品详细信息.php

<?php
include('db.php');
$prodID = $_GET['prodid'];
if(!empty($prodID)){
    /// if i uncoment this line then code work fine 
    $bd=mysqli_connect("localhost","root","","dbgadget");
    $sqlSelectSpecProd = mysqli_query($bd,"select * from products where id = 
     '$prodID'");
    $getProdInfo = mysqli_fetch_array($sqlSelectSpecProd);
    $prodname= $getProdInfo["Product"];
    $prodcat = $getProdInfo["Category"];
    $prodprice = $getProdInfo["Price"];
    $proddesc = $getProdInfo["Description"];
    $prodimage = $getProdInfo["imgUrl"];
}
?>