difference between using $HTTP_POST_VARS and $_POST for accessing POST variables

The difference between $HTTP_POST_VARS and $_POST is that one is depricated, and one is not. $HTTP_POST_VARS (and others such as GET, SERVER, ENV, COOKIE, etc.) were all renamed in PHP 4.1.0 to variables such as $_POST, $_GET, $_SERVER, $_ENV, $_COOKIE, and so on. In later versions of PHP (possibly PHP6) they will be eventually removed.

It is noteworthy to say that the $HTTP_POST_VARS array and the $_POST superglobal array are not references...that is to say, they are separate variables. Changing the value of $_POST['foo'] will not modify the value of $HTTP_POST_VARS['foo'].