Prepend/Append a PHP-include to content, with "add_filter"

Hello there,

I´d like to prepend/append dynamic info-boxes to page content. So I´ve tried the following code:

function add_infoboxes_to_content( $content ) {

    $custom_content = include('inc/infobox1.php');
    $custom_content .= $content;
    $custom_content .= include('inc/infobox2.php');
    return $custom_content;
add_filter( 'the_content', 'add_infoboxes_to_content' );

But "adding to content" doesn´t seem to work properly with PHP-includes, regardless of the content of the included files. Though the infoboxes are displayed, following errors occur:
- The boxes are strictly prepended to the content, never appended.
- For every single included file, an "1" is prepended or appended to the content. (See screenshot.) So for the example mentioned above, you can see an "1" before and one after the content. (If I append two files to the content, thus there is a "11" after the content – nevertheless the file is prepended to the content.)

If I just add text to the content, without inclusion, everything works fine. So I wonder if "add_filter" and PHP-includes at all get along with.