PHP incremntal Price Calculator based upon quantity

Well, you certainly want to loop through the products first, and then work out a total based on the quantity passed in. Something like this?

// Using PostgreSQL as an example here
$entries = pg_fetch_all(pg_query('SELECT * FROM database ORDER BY prodict, pid ASC'));

// Have the quantities ready;
$quantities = array(
    'coke' => 1024,
    'pepsi' => 512,
);

// Prepare an array to hold the total values.
$totals = array();

// Loop through the entries in the array in a non-conventional way
while($entry = current($entries)) {
    // Get the name of the product from the array
    $product = $entry['prodict'];
    // And the quantity
    $quantity = $quantities[$product];
    // Prepare a price for this product starting at zero
    $price = 0;
    // Now use a do-while to figure out the price
    do {
        // At this point '$entry' contains information about the pricing for the first 'x' quanitity
        $quantityAtThisPrice = $entry['unit'];
        // Check the price
        $priceForThisUnit = $entry['price'];
        // Check we have any quantity remaining of this product
        if($quantity > 0) {
            // Check if the quantity at this price is null or if that quantity at this price is more than what we have left
            if($quantityAtThisPrice === null || $quantityAtThisPrice > $quantity) {
                // This means the rest of the quantity is at this price
                $price += ($quantity * $priceForThisUnit);
                // No more quantity left to price
                $quantity = 0;
            }
            // Otherwise we'll add for this unit, and move on to the next
            else {
                // Add to the price
                $price += ($quantityAtThisPrice * $priceForThisUnit);
                // Subtract the quantity we just priced
                $quantity -= $quantityAtThisPrice;
            }
        }
        // Now fetch the next entry
        $entry = next($entries);
    } while ($entry['prodict'] === $product);

    // Add the calculated price to the totals array
    $totals[$product] = $price;
}

var_dump($totals);

Got a bit carried away but I think that should work.