PHP code extension to display post image thumbnails next to the Previous/Next post links in WordPress

listing-10-4.php

$previous_post=get_previous_post();
$next_post=get_next_post();
the_post_navigation(
	array(
		'next_text' => '<span class="meta-nav" aria-hidden="true">Weiter</span> ' .
		'<span class="screen-reader-text">Weiter</span> <br/>' .
		'<span class="post-title">%title</span>' . get_the_post_thumbnail( $next_post->ID, 'thumbnail' ),
		'prev_text' => '<span class="meta-nav" aria-hidden="true">Zurück</span> ' .
		'<span class="screen-reader-text">Zurück</span> <br/>' .
		'<span class="post-title">%title</span>' . get_the_post_thumbnail( $previous_post->ID, 'thumbnail' ),
	)
);