PHP代码分为外部文件包含

I have a snippet of code that works fine. But this code has some login credentials (connecting to remote server) in it. So instead of typing the login details on each page, I would like to include the details to another include file. When I do this, i include('filename.php') in the new script and for some reason that does not work. I get the following error: Fatal error: Class 'Bigcommerce\Api\Client' not found in

The include file I created looks like this:

require_once ('../bigcommerce.php');

use Bigcommerce\Api\Client as Bigcommerce;

Bigcommerce::configure(array(
        'store_url' => 'https://www.website.com',
    'username'  => 'username',
    'api_key'   => 'api_key'
));

Bigcommerce::setCipher('RC4-SHA');
Bigcommerce::verifyPeer(false);

The code file that has the code above as an include is:

<?php 

include_once 'vendor/autoload.php';
$products = Bigcommerce::getProducts();

foreach($products as $product) {
    echo $product->name;
    echo $product->price;
        echo ("<br />");
}

?>

This is the complete block of code that works in a single file:

<?php

require ('bigcommerce.php');
use Bigcommerce\Api\Client as Bigcommerce;

Bigcommerce::configure(array(
        'store_url' => 'https://www.website.com',
    'username'  => 'username',
    'api_key'   => 'api_key'
));

Bigcommerce::setCipher('RC4-SHA');
Bigcommerce::verifyPeer(false);

$products = Bigcommerce::getProducts();

foreach($products as $product) {
    echo $product->name;
    echo $product->price;
        echo ("<br />");
}
?>

How do I put this code in an external file so that I can include it throughout my site?

Thanks in advance for your help.