feat: add getEmbeddedAppUrl method

lib/ShopifySDK.php

@@ -421,6 +421,28 @@ public static function getApiUrl() {
         return self::$config['ApiUrl'];
     }
 
+    /**
+     * Returns the appropriate URL for the host that should load the embedded app.
+     *
+     * @param string $host The host value received from Shopify
+     *
+     * @return string
+     */
+    public static function getEmbeddedAppUrl($host)
+    {
+        if (empty($host)) {
+            throw new SdkException("Host value cannot be empty");
+        }
+
+        $decodedHost = base64_decode($host, true);
+        if (!$decodedHost) {
+            throw new SdkException("Host was not a valid base64 string");
+        }
+
+        $apiKey = self::$config['ApiKey'];
+        return "https://$decodedHost/apps/$apiKey";
+    }
+
     /**
      * Maintain maximum 2 calls per second to the API
      *