Moved APPROVAL_URL Constant to PayPalConstant class

- Improvement to #195

lib/PayPal/Api/Agreement.php

@@ -3,6 +3,7 @@
 namespace PayPal\Api;
 
 use PayPal\Common\PayPalResourceModel;
+use PayPal\Core\PayPalConstants;
 use PayPal\Validation\ArgumentValidator;
 use PayPal\Api\AgreementTransactions;
 use PayPal\Rest\ApiContext;
@@ -542,7 +543,7 @@ class Agreement extends PayPalResourceModel
      */
     public function getApprovalLink()
     {
-        return $this->getLink(Payment::APPROVAL_URL);
+        return $this->getLink(PayPalConstants::APPROVAL_URL);
     }
 
     /**

lib/PayPal/Api/Payment.php

@@ -4,6 +4,7 @@ namespace PayPal\Api;
 
 use PayPal\Common\PayPalModel;
 use PayPal\Common\PayPalResourceModel;
+use PayPal\Core\PayPalConstants;
 use PayPal\Rest\ApiContext;
 use PayPal\Rest\IResource;
 use PayPal\Api\PaymentHistory;
@@ -31,11 +32,6 @@ use PayPal\Validation\ArgumentValidator;
 class Payment extends PayPalResourceModel
 {
 
-    /**
-     * Approval URL for Payment
-     */
-     const APPROVAL_URL = 'approval_url';
-
     /**
      * OAuth Credentials to use for this call
      *
@@ -377,7 +373,7 @@ class Payment extends PayPalResourceModel
      */
     public function getApprovalLink()
     {
-        return $this->getLink(Payment::APPROVAL_URL);
+        return $this->getLink(PayPalConstants::APPROVAL_URL);
     }
     
     /**

lib/PayPal/Core/PayPalConstants.php

@@ -14,6 +14,11 @@ class PayPalConstants
     const SDK_NAME = 'PayPal-PHP-SDK';
     const SDK_VERSION = '0.16.1';
 
+    /**
+     * Approval URL for Payment
+     */
+    const APPROVAL_URL = 'approval_url';
+
     const REST_SANDBOX_ENDPOINT = "https://api.sandbox.paypal.com/";
     const OPENID_REDIRECT_SANDBOX_URL = "https://www.sandbox.paypal.com/webapps/auth/protocol/openidconnect";