password_hash can return null

standard/password.php

@@ -170,7 +170,7 @@ function password_get_info ($hash) {}
  * The salt option has been deprecated as of PHP 7.0.0. It is now
  * preferred to simply use the salt that is generated by default.
  * </p>
- * @return string|false Returns the hashed password, or FALSE on failure.
+ * @return string|false|null Returns the hashed password, or FALSE on failure, or null if the algorithm is invalid
  * @since 5.5.0
  */
 function password_hash ($password, $algo, $options = null) {}