diff -u b/services_api_key_auth.module b/services_api_key_auth.module
--- b/services_api_key_auth.module
+++ b/services_api_key_auth.module
@@ -83,7 +83,13 @@
   $form['api_key_source'] = array(
     '#type' => 'select',
     '#title' => t('API Key source'),
-    '#description' => t('Where look up an API key: in request parameters or in HTTP header?'),
+    '#description' => t('Where look up an API key: in request parameters or in HTTP header? Note, that <a href="@href_php_net" target="_blank">@var_request</a> variable contains the contents of @var_get, @var_post and @var_cookie.', array(
+      '@href_php_net' => 'http://php.net/manual/en/reserved.variables.request.php',
+      '@var_request' => '$_REQUEST',
+      '@var_cookie' => '$_COOKIE',
+      '@var_post' => '$_POST',
+      '@var_get' => '$_GET',
+    )),
     '#options' => array(
       'request' => t('Request (@example)', array('@example' => "\$_REQUEST['api-key']")),
       'header' => t('Header (@example)', array('@example' => "\$_SERVER['HTTP_API_KEY']")),