Added HMAC validation

Author: Robert Wittman

lib/Shopify.php

@@ -194,6 +194,24 @@ public static function __callStatic($method, $args)
         throw new Exception\ApiException("Call to undefined function {$method}");
     }
 
+    /**
+     * Verify that HMAC hash of parameters matches HMAC signature
+     * @return [type] [description]
+     */
+    public static function validateHmac()
+    {
+        $params = [];
+        foreach($_GET as $param => $value) {
+    	    if ($param != 'signature' && $param != 'hmac') {
+    		$params[$param] = "{$param}={$value}";
+    	    }
+    	}
+        asort($params);
+        $params = implode("&", $params);
+
+        return $_GET['hmac'] === hash_hmac('sha256', $params, self::$api_secret);
+    }
+
     /**
      * Return the root API url based on the authenticated store
      * @return string