PHP ile tersten yazdırma

if (!isset($_POST['goster'])) {
echo '
<form action="index.php" method="post">
<input type="text" name="yazi"><input type="submit" name="goster" value="Bak">
</form>
';
}
else {
$kelime = $_POST['yazi'];
$x = 1;
while ($x <= strlen($kelime)) {
$y .= substr($kelime,-$x,1);
$x++;
}
echo $y;
}