redirect - PHP header ('Location: ..') Not working on server

try to place the php code, at the beginning of the page, so that no output starts before,

    $host  = $_SERVER['HTTP_HOST'];
    $uri   = rtrim(dirname($_SERVER['PHP_SELF']), '/\\');
    $extra = 'index.htm';
    $final_url = 'http://'.$host.$uri.'/'.$extra;
    header('Location:'.$final_url);

Replace code..