diff --git a/captcha.api.php b/captcha.api.php
index 23ace70..187cf11 100644
--- a/captcha.api.php
+++ b/captcha.api.php
@@ -30,8 +30,17 @@
  * Let's give a simple example to make this more clear.
  * We create the challenge 'Foo CAPTCHA', which requires the user to
  * enter "foo" in a textfield.
+ *
+ * @param $op
+ *   List or generate captcha.
+ * @param $captcha_type
+ *   Captcha type.
+ * @param $captcha_sid
+ *   The $captcha_sid is a unique identifier for an instance of the captcha.
+ *   State information on each captcha session is stored in captcha_sessions,
+ *   such as is solved status.
  */
-function foo_captcha_captcha($op, $captcha_type = '') {
+function foo_captcha_captcha($op, $captcha_type = '', $captcha_sid = NULL) {
   switch ($op) {
     case 'list':
       return ['Foo CAPTCHA'];