<?php

if($submit)
{
   if($operator == '*')
   {
       echo $numa * $numb;
   } elseif($operator == '/')
   {
       echo $numa / $numb;
   } elseif($operator == '+')
   {
       echo $numa + $numb;
   } elseif($operator == '-')
   {
       echo $numa - $numb;
   }
} else { ?>
<form method="POST" action="<?php $_SERVER['PHP_SELF']; ?>">
 <input type="text" name="numa" size="10">
 <select name="operator">
 <option value="+">Add</option>
 <option value="-">Subtract</option>
 <option value="*">Multiply</option>
 <option value="/">Divide</option>
 </select>
 <input type="text" name="numb" size="10">
 <input type="submit" value="Calculate" name="submit">
</form>
<?php } ?>

Read more: http://www.webdesign.org/web/web-programming/php/php-calculator.10025.html#ixzz0TUVwwSds