Submission of form with array of input using AJAX not working

Submission of form with array of input using AJAX not working

Click handler
$('.to-pay').click(function(e)
{
var finputs=$('form.inputs-form').serialize();
alert(finputs);
$.ajax(
{
url: "URL_HERE",
type:"POST",
data: finputs,
success: function(data)
{
$('.another-div').html(data);
}
});
});
PHP part
$pid=$_POST['pid'];
$size=$_POST['product-size'];
$quantity=$_POST['quantity'];
foreach($pid as $key => $prod_id)
{
echo "This part is called";
echo $prod_id." of size ".$size[$key]." ".$quantity[$key]." numbers.";
}
The problem is in the PHP side. The foreach() part is not getting
executed. In the jQuery side I checked using alert() and the data looks
like,
pid%5B%5D=1&product-size%5B%5D=100&quantity%5B%5D=10&pid%5B%5D=2&product-size%5B%5D=150&quantity%5B%5D=20
What is the problem in the PHP side?