How do I prevent others from sending their own data to my php page?

Question

Suppose I have a registration page in my website that contains a registration form:

Answer 1

After submitting the form, the request goes to a php page. In that page you use the below code to check if its coming from your domain:

if(parse_url($_SERVER['HTTP_REFERER'], PHP_URL_HOST) == "yourdomain.com") {

     //process the request
} else {

     echo "Sorry";

}

EDIT: Agree. This is not secure. Just a bad hack