Update Form Variable Then Send to PHP Server

Hi,

I think that what I want to do is probably pretty easy, but I'm just learning PHP so it's not so easy  for me to understand why it doesn't work.

I have a form which posts back to itself:

echo "<form method='POST' name='form1' action=".$_SERVER['PHP_SELF'].">";

On that form I have a field defined like this:

echo "<input type='text' name='INCASE' size='13'>";

and two hidden fields defined like this:

 echo "<input type='hidden' name='_submit_check' value='1'/>";

echo "<input type='hidden' name='FirstCase' value=".$OUTCASRECORD[0]["XCASE"].">";

What happens is that the user enters a case number into the input field..clicks the Submit button and then I use the technique:

 if (array_key_exists('_submit_check', $_POST)) {

to see if the form has been submitted.  If it has,  I do some PHP magic on the server and get an array of data back which is contained in $OUTCASRECORD in this example.

I take the first element of $OUTCASRECORD and load that into the hidden field FirstCase, build a table with the rest of the data and then display the form to the user.

That all works great.

Now...here's the tricky part that I can't seem to figure out:

I have another button on my form of type "submit" and when the user hits that one,  what I want to have happen is that I want the input field INCASE to contain the value in the hidden field FirstCase.    I then want to submit the form at that time so that when it goes back to my PHP server,  that field, INCASE now contains the value in FirstCase.

I've tried some javascript and tried doing this in a PHP function but either neither of those techniques are correct or else I'm just not coding them correctly (which is most likely).

Can someone offer a suggestion on how to do this in PHP?    Hopefully I have explained the setup and what I want to do clearly.

Thanks