session_start() [function.session-start]

PHP
1

please i am having this error, wat does it mean.
Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/reachea2/public_html/activate.php:2) in /home/reachea2/public_html/include/global.php on line 7


<?php
 //ob
    ob_start();

    //session
    session_start();

    //connect to database
    $error = "Problem connecting";
    mysql_connect("localhost","12345","abcdef") or die($error);
    mysql_select_db("12345") or die($error);

    ?>

and

<?php
include ‘include/global.php’;

$code = $_GET['code'];

if (!$code)
    echo "No code supplied";
else
{
    $check = mysql_query("SELECT * FROM users WHERE code='$code' AND active='1'");
    if (mysql_num_rows($check)==1)
        echo "You have already activated your account";
    else
    {
        $activate = mysql_query("UPDATE users SET active='1' WHERE code='$code'");
        echo "Your account has been activated!";
    }
    
}
?&gt;
2

there was white space before the code, i have sorted it