Consider `form.php` and `process.php`. What captcha does is to check if the form has been submitted from `form.php` or directly `POST`ed to `process.php`. A simple way is to check the `Referrer` and prevent processing if the actuall `Referrer` is not `form.php`.

What can be the main problem for this method, as it is not normally used?

Is it possible for a spammer bot to send the form from `form.php` to have it as the `Referrer`?

Can this method slightly stop spam posts (if not completely)?