$a (int) $_POST['a'];
$a1 = (int) $_POST['a1'];
$a2 = (int) $_POST['a2'];
$a3 = (int) $_POST['a3'];
$a4 = (int) $_POST['a4'];

$toplam = $a + $a1 + $a2 + $a3 + $a4;
echo $toplam;