POST random variable

I have a form with checkboxes.. and I need to send one at random as a variable.

Check more checkboxes and write one at random:

<form action="index.php" method="POST">
 <input type="text" name="name" placeholder="Jméno vyvolávače" maxlength="32"><p>
 <table>
  <tr>
   <input type="checkbox" onClick="toggle(this)" /> Vyber vše<br/>
   <td>Ahri<input type="checkbox" name="champ" value="Ahri"></td>
   <td>Aatrox<input type="checkbox" name="champ" value="Aatrox"></td>
   <td>xx<input type="checkbox" name="champ" value="xx"></td>
   <td>xx<input type="checkbox" name="champ" value="xx"></td>
  </tr>
 </table>
 <input type="submit">
</form>
<?php
 echo "Vyvolávači ".$name," pickni si ".$champ," na ".$line;
?>

So now I check Ahri and Aatrox.. and page submits one at random. How can I do this?