amount variable value changes in paypal while payment begins - php

Question

I am using paypal functionality in a sandbox environment.I am implementing very basic functionality of paypal. The problem is that when user buys more than 1 quantity of same product the value of amount in the paypal gets multiplied. I am sure that I am missing some variable to use but can't figure out that. The total amount I am calculating on the server side. e.g if buyer selects 2 quantities of same product, the price get multiplied 2 times and this is the value of amount variable in paypal form. But when form is submitted to paypal, the amount variable's value become the value of item price and final amount in paypal is again multiplied by 2.
I am giving here my paypal form
<form action="<?php echo $paypal_url; ?>" method="post" name="frmPayPal1">
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="xyz#exmp.com">
<input type="hidden" name="item_name" value="<?php echo $_SESSION['itemName'];?>">
<input type="hidden" name="quantity" value="<?php echo $quant;?>">
<input type="hidden" name="item_price" value="<?php echo $price;?>">
<input type="hidden" name="amount" value="<?php echo $z?>">
<input type="submit" name="submit" value="pay">
<INPUT TYPE="hidden" NAME="return" value="return website">
<INPUT TYPE="hidden" NAME="cancel_return" value="return website">
</form>
where $quant is the quantity of product, $price is the price of product,
$z is the calculated total amount($quant * $price)
if $quant=2
$price=1000 per unit of product
then $z = 2000
and in paypal website the values become
Item price: 2000.00
Quantity: 2
Amount: 4000
please if any body can help.

Answer 1

According to the Paypal manual there's no variable named item_price.
There's quantity:
Number of items.
And there's amount:
The price or amount of the product, service, or contribution, not
including shipping, handling, or tax
So you should have:
$z = 1000; //price_per_product;
$quant = 2;