As soon as a form is submitted, the variable is available in PHP.
For example:
<html>
<h1>You typed <? print $_GET['test']; ?></h1>
<form method="GET">
<input type="text" name="test">
<input type="submit" name="Submit!">
</form>
</html>
The same principle holds true when passing to another page. All values can be accessed through $GET and $POST.
OHB