How do I add thumbnail to current Prev and next buttons in posts?

Question

The current Prev and Next buttons in a wordpress theme that I'm using is rendered from this function in functions.php:

if ( !function_exists( 'krunk_post_navigation' ) ) {
    function krunk_post_navigation( $direction, $posting, $term ) {

        $post_navigation = '';

        if( $posting ) {

            $post_navigation .= '<a class="post-navi-' . $direction . ' clearfix" href="' . get_permalink( $posting->ID ) . '">' .
                                    '<div class="post-navi-inner">';
                                        $post_navigation .= '<div class="post-navi-' . $direction . '-info">' .
                                            '<div class="table-cell-middle">' .
                                                '<div class="post-navi-label"><i class="fa fa-long-arrow-left"></i>' . esc_attr( $term ) . '<i class="fa fa-long-arrow-right"></i></div>' .
                                            '</div>' .
                                        '</div>' .
                                    '</div>' .
                                '</a>';


        }

        return $post_navigation;
    }
}

How do I integrate the following code to the function above so that there will be a thumbnail of the previous and next posts too along with the arrows?

$previousPost = get_previous_post();
$previousThumbnail = get_the_post_thumbnail( $previousPost->ID );
$nextPost = get_next_post();
$nextThumbnail = get_the_post_thumbnail( $nextPost->ID );

Answer 1

Without more context it's hard to be sure, but it looks like this function is being called twice: once for each link ($direction="next" and $direction="previous"). The $posting parameter looks to be the next or previous post. If my assumptions are correct, you need to get the thumbnail for that post:

$thumbnail = get_the_post_thumbnail( $posting->ID );

and then insert it into the $post_navigation string being created. I would put it at the end of line 8 and see how that works:

$post_navigation .= '<a class="post-navi-' . $direction . ' clearfix" href="' . get_permalink( $posting->ID ) . '">' . $thumbnail .

Note that get_the_post_thumbnail() will return an image sized to the 'post-thumbnail' dimensions, and will likely be too large. So you may need to define a new size and pass that as a parameter to get_the_post_thumbnail(). You will also probably need CSS to style the link to accommodate the image.