More rules apply than you may think. You need to know the difference between certain symbols too.
Here are the sources used in this tutorial:
simple.php
<?php define("HAPPY", 22); if(isset($_REQUEST['s'])){ $simple = (int)$_REQUEST['s']; }else{ $simple = 0; } $tires = $simple*HAPPY; $tires = 1.06*$tires; echo "You need to pay us $$tires in cash today for your $simple tires."; ?>