errore nel creare una cartella

[rb10]

function registerNewUser($username, $password, $password2, $email)
{
 
    global $seed;
 
    if (!valid_username($username) || !valid_password($password) || 
            !valid_email($email) || $password != $password2 || user_exists($username) || mail_exists($email))
    {
        return false;
    }
 
   mkdir("{$_SERVER['DOCUMENT_ROOT']}upload/$username", 0777);

    $code = generate_code(20);
    $sql = sprintf("insert into login (username,password,email,actcode) value ('%s','%s','%s','%s')",
        mysql_real_escape_string($username), mysql_real_escape_string(sha1($password . $seed))
        , mysql_real_escape_string($email), mysql_real_escape_string($code));