How to make the form generate fields for user input

Firstly, I must say hi to everyone here, and thanks in advance for trying to help me!

I am a newbie to java/php programming, so..

I am trying to have a form that can change quantities. For example, the user needs to order 5 items, and when he choose 5, and click on change quantity, the script will generate 5 fields for him to fill.

An example is as follows..

http://spreesg.com/b2b.php

I am really thankful if any of you can help me. Thanks!

Regards,
Shawn