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