cart.php

<?php

$cart = [
    "product_id_3" => 8,
    "number_of_quantity_3" => 3,
    "price_3" => 30000,
    "total_3" => 90000,
    "product_id_2" => 6,
    "number_of_quantity_2" => 3,
    "price_2" => 100,
    "total_2" => 300,
    "product_id_1" => 5,
    "number_of_quantity_1" => 3,
    "price_1" => 100,
    "total_1" => 300,
    "discount" => 0,
    "grand_total" => 90600
];

//$cart = $_POST;

$discount = $cart['discount'];
$grandTotal = $cart['grand_total'];

unset($cart['discount'], $cart['grand_total']);


$keys = array_keys($cart);

$ids = [];
foreach ($keys as $key) {
    $seperateKeys = explode("_", $key);
    $ids[] = $seperateKeys[array_key_last($seperateKeys)];
}

$ids = array_unique($ids);

$query = "INSERT INTO sales_total(`discount`,`grand_total`) VALUES($discount, $grandTotal)";

echo $query . PHP_EOL;

foreach ($ids as $id) {
    $product_id = $cart["product_id_{$id}"];
    $quantity = $cart["number_of_quantity_{$id}"];
    $price = $cart["price_{$id}"];
    $total = $cart["total_{$id}"];

    $query = "INSERT INTO sales(`product_id`,`number`,`price`,`total`) VALUES($product_id, $quantity, $price, $total)";

    echo $query . PHP_EOL;
}

The output:

INSERT INTO sales_total(discount,grand_total) VALUES(0, 90600)
INSERT INTO sales(product_id,number,price,total) VALUES(8, 3, 30000, 90000)
INSERT INTO sales(product_id,number,price,total) VALUES(6, 3, 100, 300)
INSERT INTO sales(product_id,number,price,total) VALUES(5, 3, 100, 300)