<?php
namespace WoowUp;

use WoowUp\Endpoints\Products;
use WoowUp\Endpoints\Purchases;
use WoowUp\Endpoints\Users;

class Client
{
    protected $http;

    public $purchases;
    public $mailings;
    public $users;
    public $segments;
    public $products;

    public function __construct($apikey, $host = 'https://api.woowup.com', $version = 'apiv3')
    {
        $url = $host . '/' . $version;

        $this->purchases = new Purchases($url, $apikey);
        $this->users     = new Users($url, $apikey);
        $this->products  = new Products($url, $apikey);
    }
}

<?php
namespace WoowUp;

use WoowUp\Endpoints\Purchases;
use WoowUp\Endpoints\Users;
use WoowUp\Endpoints\Products;

class Client
{
    const HOST    = 'https://api.woowup.com';
    const VERSION = 'apiv3';

    protected $http;

    public $purchases;
    public $mailings;
    public $users;
    public $segments;
    public $products;

    public function __construct($apikey)
    {
        $this->purchases = new Purchases(self::HOST . '/' . self::VERSION, $apikey);
        $this->users     = new Users(self::HOST . '/' . self::VERSION, $apikey);
        $this->products  = new Products(self::HOST . '/' . self::VERSION, $apikey);
    }
}

--- 
+++ 
@@ -1,15 +1,12 @@
 <?php
 namespace WoowUp;
 
+use WoowUp\Endpoints\Products;
 use WoowUp\Endpoints\Purchases;
 use WoowUp\Endpoints\Users;
-use WoowUp\Endpoints\Products;
 
 class Client
 {
-    const HOST    = 'https://api.woowup.com';
-    const VERSION = 'apiv3';
-
     protected $http;
 
     public $purchases;
@@ -18,10 +15,12 @@
     public $segments;
     public $products;
 
-    public function __construct($apikey)
+    public function __construct($apikey, $host = 'https://api.woowup.com', $version = 'apiv3')
     {
-        $this->purchases = new Purchases(self::HOST . '/' . self::VERSION, $apikey);
-        $this->users     = new Users(self::HOST . '/' . self::VERSION, $apikey);
-        $this->products  = new Products(self::HOST . '/' . self::VERSION, $apikey);
+        $url = $host . '/' . $version;
+
+        $this->purchases = new Purchases($url, $apikey);
+        $this->users     = new Users($url, $apikey);
+        $this->products  = new Products($url, $apikey);
     }
 }
Add host and version as parameters in client constructor
