'Cannot modify header information' error

[NoBullMan]

Hi,
when I click on a link ("Add to cart"), I go to a file where I check if the item clicked was the first item to be added to the cart; If so, I would redirect the user to a "information form".

In the items for sale page, for one of the items, I have:

session_start();
....
<a href="../../shopping/add_cart.php?id=4171&builder=1>Add to cart</a>

Then, in add_cart.php:

  session_start();

  if (isset($_GET['id']))
    @ $id = $_GET['id'];
  if (isset($_GET['builder']))
    @ $builder = $_GET['builder'];

echo 'some debugging line of code';
// if the first item is being added to cart
  if (!isset($_SESSION['builder'])) { 
    $_SESSION['builder'] = $builder;
    header("Location: info_form.php?builder=$builder"); // redirect to information form
  }

Here, I get the following warning on the page, after clicking on the "Add to cart" for the first time:

Warning: Cannot modify header information - headers already sent by (output started at /home3/www/mydomain/shopping/add_cart.php:11) in /home3/www/mydomain/shopping/add_cart.php on line 15

where line 15 is the "header" statement.
On the page that list the items for sale, I also have a session_start() on the top.

Thanks a lot.

[laserlight]

It's quite a common error, and is caused by writing output before (in your case) a header() call.

The most obvious culprit is:
echo 'some debugging line of code';

[NoBullMan]

Thanks alot. That fixed the problem. So, I cannot output anything before a call to header(). I guess once a header is sent (when I did an "echo ....", I was not allowed to send another header.
Why?

Thanks.