set php radio button checked by default
Here you go:
<li><input type="radio" name="r1" value="o" onClick="submit();" <?php echo ($_SESSION['r1'] != "p") ? 'checked="checked"' : ''; ?> />On</li><li><input type="radio" name="r1" value="p" onClick="submit();" <?php echo ($_SESSION['r1'] == "p") ? 'checked="checked"' : ''; ?> />Off</li>