POST random variable

I have form with checkboxs.. and I need to send one random variable (sorry for my english #NoHatePlease) Check more checkboxs and write one 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 write one random.. How can i do this?