$items = [
    0 => ['price' => 100, 'tax' => 8],
    1 => ['price' => 100, 'tax' => 18],
    2 => ['price' => 100, 'tax' => 15],
];

$toplamVergi = array_reduce($items, function($prev, $item){
    return $prev + ($item['price'] * $item['tax'] / 100);
});

echo $toplamVergi;