index.php
Всего: <?php echo (isset($_SESSION['amount']) ? $_SESSION['amount'] : 0); ?>
<button onclick="location.href='add.php?amount=15'">добавить 15</button>
<button onclick="location.href='add.php?amount=50'">добавить 50</button>
________________________________________________________________
add.php
<?php
if (isset($_GET['amount'])) {
    session_start();
    $_SESSION['amount'] += (int) $_GET['amount'];
}
Header('Location: index.php');
?>