I need to change my submit button class

I need to change form left to right layout and right to left when i press the button here is my code :

<form method="post">
<ul>
 <li class="left">
    <input name="left" type="submit" class="left" value="left">
 </li>
 <li class="right">
  <input name="right" type="submit" class="right" value="right">
 </li>
 </ul>
</form>

now I need to add an if statement like :

if(isset($_POST['left'])) {
echo '    <ul>
 <li class="left">
    <input name="left" type="submit" class="left" value="left">
 </li>
 <li class="right">
  <input name="right" type="submit" class="right" value="right">
 </li>
 </ul>';
}
if(isset($_POST['right'])){
{
echo '    <ul>
 <li class="left">
    <input name="left" type="submit" class="right" value="left">
 </li>
 <li class="right">
  <input name="right" type="submit" class="left" value="right">
 </li>
 </ul>';
}