Someone

 <?php

  if(isset($_POST['act']) 

     && !empty($_POST['z1']) 
     && !empty($_POST['z2'])
     && is_numeric($_POST['z1']) 
     && is_numeric($_POST['z2']))
 {
     if($_POST['act'] == 'add')

     {
         $calc = ($_POST['z1']+$_POST['z2']);
         $op = '+';
     }

     if($_POST['act'] == 'sub')

     {
         $calc = ($_POST['z1']-$_POST['z2']);
         $op = '-';
     }
     if($_POST['act'] == 'div')
     {
         $calc = ($_POST['z1']/$_POST['z2']);
         $op = '/';
     }
     if($_POST['act'] == 'mul')
     {
         $calc = ($_POST['z1']*$_POST['z2']);
         $op = '*';
     }
     if($_POST['act'] == 'pro')
     {
         $calc = (($_POST['z2']*$_POST['z1'])/100);
         $op = '% of';
     }
     echo '
     <p>'.$_POST['z1'].' '.$op.' '.$_POST['z2'].' = '.$calc.'</p>
     <p><a href="./">try again</a></p>';

  
 }else{
     echo '
     <form method="post" action="./">
         <input type="text" name="z1" size="5" /> 
         <select name="act">
             <option value="add">[ + ]</option>
             <option value="sub">[ - ]</option>
             <option value="div">[ / ]</option>
             <option value="mul">[ * ]</option>
             <option value="pro">[ % ]</option>
         </select>
         <input type="text" name="z2" size="5" /> 
         <input type="submit" name="doit" value="calculate" />
     </form>';
 }
     
 ?>