CONVERTOR CM TO DM ETC

Hello, i have a problem with my php code, im student of the high school and we just started doing the php and i would like to do convertor something like cm to dm etc. I started doing some php and here is it, i want someone who has a time to help me do the whole convertor.. here is my code:

<form method="post">
 <select name="z">
       <option name="Vyber"  value="">Vyber</option>
       <option name="cm" value="cm">cm</option>
       <option name="dm"  value="dm">dm</option>
    <option name="m"  value="m">m</option>
     </select>
  <select name="do">
       <option name="Vyber2"  value="">Vyber</option>
       <option name="cm2"  value="cm">cm</option>
       <option name="dm2"  value="dm">dm</option>
    <option name="m2"  value="m">m</option>
     </select>
   <input type="submit" name="submit" value="GO!" />
</form>



<?php

 if(isset($_POST['submit'])){
 $selected1 = $_POST['z'];
 $selected2 = $_POST['do'];
 echo "Z " . $selected1 . ":" . "<form name='final' method='post'><input type='text' />" . "<br>";
 echo "Do " . $selected2 . ":" . "<input type='text' />" . "<br>";
 echo "<input type='submit' value='Vypocitat!' /></form>";
 }

  if (isset($_POST['cm'], $_POST['dm2'])){
  echo $_POST['cm'] * 10;
 }

?>