I found the problem...
It's my double call for print function...
at first I write the code like this :
$content = print '<iframe src="../pengguna/daftar_pengguna.php" overflow="hidden" width="1074" height="500"></iframe>';
then I call the $content.
It solved when I write the code like this :
$content = '<iframe src="../pengguna/daftar_pengguna.php" overflow="hidden" width="1074" height="500"></iframe>';
[edit]Code block fixed[/edit]