Problem with sending "SetCookie" first in php code

According to this manual: http://us2.php.net/setcookie I have to set the cookie before anything else.

Here is my cookie code:

if (isset($_COOKIE['watched_ads'])){
    $expir = time(开发者_StackOverflow中文版)+1728000; //20 days
    $ad_arr = unserialize($_COOKIE['watched_ads']);
    $arr_elem = count($ad_arr);
    if (in_array($ad_id, $ad_arr) == FALSE){
        if ($arr_elem>10){
        array_shift($ad_arr);
        }
        $ad_arr[]=$ad_id;
        setcookie('watched_ads', serialize($ad_arr), $expir, '/');
    }
}
else {
    $expir = time()+1728000; //20 days
    $ad_arr[] = $ad_id;
    setcookie('watched_ads', serialize($ad_arr), $expir, '/');
}

As you can see I am using variables in setting the cookie.

The variables comes from a mysql_query and I have to do the query first. But then, if I do, I will get an error message:

 Cannot modify header information - headers already sent by ...

The error points to the line where I set the cookie above.

What should I do?

UPDATE:

I do this before the setCookie part:

$ad_id=$_GET['ad_id'];
$query2 = "SELECT * FROM classified WHERE classified.ad_id = '$ad_id'";
$results2 = mysql_query($query2) or die(mysql_error());
$row2 = mysql_fetch_array($results2);
$cat = $row2['category'];
$action=$row2['action'];
$sql_table='';
$num_rows = mysql_num_rows($query_results);
if ($num_rows != 0){
   HERE COMES THE SETCOOKIE PART

As others have suggested, make sure, you are not outputting any html or whitespace before you set your cookie.

This will fail because you are printing html before you set your cookie.

<p>
<?php
  // your cookie code - note <p> tag before <?php tag
  // ...
?>

This will also fail, because you are printing whitespace before you set your cookie.

 
<?php
  // your cookie code - note the extra linebreak before <?php tag
  // ...
?>

Also

 <?php
  // your cookie code - note the extra space before <?php tag
  // ...
?>

If you use an UTF encoding for your php script (and if you are not in one of the english speaking countries, chances are that you do), make sure your editor is set that it does not include byte order mark (BOM) at the begining of every file. See http://en.wikipedia.org/wiki/Byte_order_mark for more detail on BOM.

The restriction is not that you must not do anything before setting your cookies, merely that you must no output anything before setting your cookies.

For example, let's say we want to get some data from the database, output it to the user and set it to the cookie.

<?php
$data = getDbData();
echo $data['field'];
setcookie('field', $data['field'], time()+86400, '/');

This will fail because we've output the data before setting the cookie. We can fix it by moving the output to after we set the cookie.

<?php
$data = getDbData();
setcookie('field', $data['field'], time()+86400, '/');
echo $data['field'];

Make sure that you do not print anything prior to adding header-based information (as cookies are).

I can't see any problems with the code, unless mysql outputs an error, which could cause this.

This is a shot in the dark, but make sure you don't have any whitespace (or anything else for that matter) before the opening php tags. Also make sure you don't have any trailing whitespaces after the closing php tags in the files you include.

The error message you showed us says that the headers were sent on the setcookie() line. Thus, you may be setting headers or cookies later in the code, which is causing the error. (Or so I believe, since I can't recall the error word for word and you cut it off at the critical point)