As you said there, i agree with your opinion to use $_SESSION['id'], but i just thinking, if someone have logged in,....he still use the same browser, and then he opens his own php form that he had modified,....so he can use his own form & still have their session id also,....so he still can attack the website,.....is this right what i'm talking about????
Well, you could add that in for extra security, because there's ways around everything. Sessions aren't like cookies.. I mean they are, but I think what you're thinking in terms of functionality is a cookie. The user wouldn't have that session on another website, just that one.
Ive added reCAPTCHA to the registration page, would this stop people from being able to spam it from remote forms?