D7: How to deal with the variables visibility? Now I have some variables collected from POST

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


I need to use them in 2 functions: hook_form() and the callback mail_send() the function that format and sends my email with form elements.

I have implemented the following hooks:

function MYMODULE_page() {...}
function MYMODULE_form() {...}
function MYMODULE_form_submit($form, &amp;$form_state) {
mail_send($form_state['values']);
}

//a custom callback mail function call in the submit.
function mail_send($form_values) {}


...

Now I don't know where to place those variables to have maximum visibility: 


if I place them in MYMODULE_form() the show up in the form but they are not available in the mail_send().
if I place them outside everything they are not available inside functions.
if i place them inside a hook_init() the same as above.


I need a place where all variables are visible. Thanks
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.