如何将动态2d数组发布到服务器

Question

I am a beginner of PHP. I am trying use Codeigniter to implement a simple shopping cart, but I got stuck by a problem: I wanna update cart content, but I don't know how to retrieve data from client side, the data describes several pair of rowid and quantity of product.

My code is as following:

javascript:

$(document).ready( function(){

$("#btn .update").click(function(){
    var cell;
    var data = [];
    var rowNo = document.getElementsByTagName("tr").length -2;  // exclude head and bottom

    var i;
    for(i = 1; i <= rowNo; i++){
        data[i-1] = [];
        cell = document.getElementsByName(i+"[rowid]");
        data[i-1][0] = cell[0].value;   //rowid
        cell = document.getElementsByName(i+"[qty]");
        data[i-1][1] = cell[0].value;   //quantity
    }

    $.post('http://localhost/CodeIgniter/shopping_cart/update_cart', data, function()
    {
            return false;
    });
}); 
});

shopping_cart.php:

function update_cart(){
    $data = $this->input->post();
}

I couldn't get any data from post. Is there any other way I can retrieve the data or implement this function?