You have to pass the paramaters back, probably using POST. Assuming you're using a form, then basically all your form fields (textboxes etc) will have names and you simply POST them when you click back and then have the fields echo the value of the post.
So if you had a form:
<form action="process.php" method="post">
<input type="text" size="25" name="searchtype" />
<input type="text" size="25" name="searchterm" />
</form>
You could grab the values like this:
$searchtype = $_POST['searchtype'];
$searchterm = $_POST['searchterm'];
And then you could also display them on the back by changing:
<input type="text" size="25" name="searchterm" />
to:
<input type="text" size="25" name="searchterm" value=" <? echo($searchterm) ?> " />
Hope this helps!