I assume from your description you have something like
<form name = "form1" action="myscript.php" method="get">
<select name="choice" onchange="updateOtherField(this.form)">
<option value="1">Choice 1</option>
<option value="2">Choice 2</option>
<option value="3">Choice 3</option>
</select>
<input type="text" name="otherField" value="">
<input type="submit" value="Submit>
Am I close?
If so, in myscript.php
$choice = $_GET["choice"];
$otherField = $_GET["otherField"];
Both the above should have the selected value.
hth