Create a custom post navigation in WordPress

gistfile1.txt

<?php
add_shortcode( 'fs_prev_next_thumbs', 'generate_faq_landing_link_shortcode' );
function generate_faq_landing_link_shortcode( $atts, $content ) {
 
	global $post; // if outside the loop
	
	$prev_next = '';

	$prev_post = get_previous_post();
	$next_post = get_next_post();
	
	// wrapper opening class
	if($prev_post && $next_post) {
		$prev_next_wrapper_class = 'fs-post-nav-thumb-both';;
	}
	else {
		$prev_next_wrapper_class = 'fs-post-nav-thumb-single';
	}
	
	// wrapper opening
	$prev_next .= '<div class="fs-post-nav-thumb-container ' . $prev_next_wrapper_class . '">';
	
	if($prev_post) {
		$prev_title = strip_tags(str_replace('"', '', $prev_post->post_title));
		$prev_featured_img_url = get_the_post_thumbnail_url($prev_post->ID,'full'); 
		$prev_next .= '<div class="fs-post-nav-thumb"><a rel="prev" href="' . get_permalink($prev_post->ID) . '" title="' . $prev_title. '" style="background-image:url('. $prev_featured_img_url .'); background-position: center; background-size: cover;"><div><p class="fs-thumb-prev">Prev</p><p class="fs-thumb-title">' . $prev_title . '</p><p class="fs-thumb-excerpt">' . get_the_excerpt($prev_post->ID) . '</p></div></a></div>';
	}
	if($next_post) {
		$next_title = strip_tags(str_replace('"', '', $next_post->post_title));
		$next_featured_img_url = get_the_post_thumbnail_url($next_post->ID,'full');
		$prev_next .= '<div class="fs-post-nav-thumb"><a rel="next" href="' . get_permalink($next_post->ID) . '" title="' . $next_title. '" style="background-image:url('. $next_featured_img_url .'); background-position: center; background-size: cover;"><div><p class="fs-thumb-next">Next</p><p class="fs-thumb-title">' . $next_title . '</p><p class="fs-thumb-excerpt">' . get_the_excerpt($next_post->ID) . '</p></div></a></div>';
	}
	
	// wrapper closing
	$prev_next .= '</div>';
	
	// begin output buffering
	ob_start();
	 
	echo $prev_next;
	
	//get the buffer contents
	$content = ob_get_contents();
	
	//end the output buffer
	ob_end_clean();
	
	return $content;
	
} ?>