How to send new variable $total back to be displayed into an <input ="text"> box

[phpspaz]

Hi, I have a form  that is processed by PHP script $_REQUEST.

 

the pages work when its only:  echo (to a new page)

But, I'd like it to be put into a read only text box in a table.

 

The basics of this would be to select products from drop down menus , the php script processes their values, and then i would

like that $total , to be returned to an:

 

<input name="whatever_subtotal" type="text" size="5" readonly="readonly">

 

How would i add this into the php script, and HTML page already set up? 

Thanks for the help.  ???