A hacky example of something I'm actually doing in a client project. We need to include the content of sub posts/pages on a parent post/page using the `the_content` filter. We also need to pass the content of the child posts/pages through the `the_content` filter, so it's important to remove the filter immediately otherwise it's recursively appl…

gistfile1.php

<?php

add_filter( 'the_content', 'my_content_filter' );

/**
 * Auto appends the content of child posts to the main post
 */
function my_content_filter( $content ) {
    global $post;

    if ( ! is_object( $post ) || ! is_singular( 'post' ) || 0 != $post->post_parent ) {
        return $content;
    }

    // Remove filter so it's not added to our child posts as well
    remove_filter( 'the_content', 'my_content_filter' );

    // Set up child posts
    $children = get_children(
        array(
            'post_type'   => 'post',
            'post_parent' => $parent->ID
        )
    );

    if ( count( $children ) > 0 ) {
        foreach( $children as $child ) {
            $content .= '<br />' . apply_filters( 'the_content', $child->post_content );
        }
    }

    return $content;
}
?>