add_filter adds output in the head

Filters should return their content, and not echo it. That is why you see the output in your head as well


I really have no idea why you would want to use require or include in your filter.

Here is an example of your filter should be coded

add_filter( 'the_content', myFunction );
function myFunction( $content )
    $options = get_option('myPlugin_settings');
    if ( is_page( $options['myPlugin_list'] ) ) {
        return "--------------------------------------------------------";
    return $content;