如何纠正在PHP中使用命名空间

I'm trying to use the PHP zendesk API in my application. It's in folder

Library/Zendesk/API

The classes are namespaced:

namespace Zendesk\API;

To invoke I use:

use Library\Zendesk\API\Client as ZendeskAPI;
$client = new ZendeskAPI($subdomain, $username);

Which gives a class not found error (checked - Zendesk\API\Client.php is included). It's fixed when I edit Zendesk\API\Client.php and change namespace to

namespace Library\Zendesk\API;

Doesn't seem like the right way to go. Any tips on the proper way to handle this?

From PHP documentation how to correctly use namespace and namespace nested

the correct format is with \

namespace MyProject\Sub\Level;