consider this simple example
test.php
<frameset rows="200,*">
<frame src="top.php<?php if (isset($_POST['var'])) {echo '?var=' . $_POST['var'];} ?>">
<frame src="bottom.php<?php if (isset($_POST['var'])) {echo '?var=' . $_POST['var'];} ?>">
</frameset>
top.php
<form action="test.php" method="POST" target="_top">
var: <input type="text" name="var"><br>
<input type="submit" name="submit" value="submit">
</form>
<?php
if (isset($_GET['var'])) {echo 'you entered: ' . $_GET['var'];}
?>
bottom.php
<?php
if (isset($_GET['var'])) {echo 'you entered: ' . $_GET['var'];}
?>