KorLagar

<?php

function total($array = []): int {
    $total = 0;
    foreach ($array as &$value) {
        $total += $value['quantity'];
    }
    return $total;
}

echo total([
    [
    'position' => 'книга по php',
    'quantity' => 1,
    ],
    [
    'position' => 'Мышь беспроводная',
    'quantity' => 12,
    ]
]);