For the maximum visibility use global variables like below

$a = $_POST['a'];
$b = $_POST['b'];
global $c;
$c = $a * $b

and you can anywhere like below,

function mail_send($form_values) {
  global $c;
  print_r($c);
}

For the maximum visibility use global variables like below

$a = $_POST['a'];
$b = $_POST['b'];
global $c;
$c = $a * $b

and you can anywhere like below,

function mail_send($form_values) {
  global $c;
  print_r($c);
}

There is no special methods exists in drupal, it is just like php.