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I have written a function for integration of PunBB login in another website. Idea is to use login infos to login into punBB
It used to work but not anymore. Here is the function :

define('PUN_ROOT', 'modules/forum/');
require PUN_ROOT.'include/common.php';

$mysql = mysql_connect("HOST", "USER", "PASS");
mysql_select_db( "MYWEB" );

$result_pun = mysql_query("select `id` from `forum_users` where `username`='$userx' ");
$row_pun = mysql_fetch_array($result_pun);
pun_setcookie ($row_pun['id'], pun_hash("$passwordx"), time() + 86400);

I get the error :

///login of a user into punBB forum
Warning: Cannot modify header information - headers already sent by (output started at /home/sparta/www/htdocs/marsweb/ForumLogin.php:3) in /home/sparta/www/htdocs/marsweb/modules/forum/include/functions.php on line 174

And I cannot login...
Any ideas or suggestions ?