Simple problem but idk how to fix it

[Dave80]

Hello

It's very simple to explain. I want to include my custom php header on my forum so i set a plugin with global_start with this code:

ob_start();
   include('custom/header.php');
   $custom_header = ob_get_contents();
ob_end_clean();

Then i call $custom_header in template. It is not working. I can see nothing so, temporarily, i'm using

$custom_header = include('custom/header.php');

It works but, as soon as i open a single <div></div> tag in header.php all my forum become totally bugged. Oh and i also tried to ob_start in my file with this in header.php:

<? ob_start('cache_page'); ?>
<div class="blablablablablablabla"></div>
<? ob_end_clean(); ?>

Again bug is served. Any idea to fix this simple problem?

Thank you!