Problem with PHP script

I have a PHP Script for user registration having a form whose method attribute is set to post and has ACTION=”” . The name of the submit button is “submit_button”.The form is intended to collect information from online users and post the same in a table called register built in MYSQL. The problem is that on submitting the form i am getting an error of ‘Undefined variable “submit_button”. I am using PHP4. The same script was working fine with php3. I have also tried using $_POST[“submit_button”]. But that too did not work.
Please help.
Thanx
Anna