Open your PHP template and save it as phparithmetic.php.

You already saw one example of arithmetic so you may realise that you can take a value, do something mathematical to it and store the answer in a variable.  Here is another example which you should paste inside the PHP tag:

$firstvalue=4;
$secondvalue=7;
$answer=$firstvalue+$secondvalue;
echo "<p>The answer is {$answer}</p>";

Upload and try.  Then replace the + operator with at least one of these to try: