WordPress function to get adjacent gallery images

sjf_deh_get_adjacent_image_link.php

<?php

/**
 * Return a link to the next/prev post in a gallery
 * 
 * @param  string $next_prev Specify if the function should return the value for the next or the prev post
 * @return string A link to the next/prev post in a gallery
 */
function sjf_deh_get_adjacent_image_link( $next_prev = 'prev' ){
	
	// The current post.
	$post = get_post();
	
	// If the post has no parent, bail.
	$post_parent = absint( $post->post_parent );
	if( empty( $post_parent ) ) { return false; }

	// If the post is not an attachment, bail.
	if( $post->post_type != 'attachment' ) { return false; }

	// Args for a get_posts call to get siblings of the current post.
	$args = array(

		// The parent of the current post.
		'post_parent' => $post->post_parent, 
		
		// So as to allow attachments.
		'post_status' => 'any',
		'post_type' => 'attachment',
		'post_mime_type' => 'image/jpeg',
		
		// Order by recency.
		'order' => 'DESC', 
		'orderby' => 'date',

		// Get all posts.
		'posts_per_page' => -1,
	);

	// Get the posts as an array, and convert it to a numerically indexed array so we can grab the next/prev
	$attachments = array_values( get_posts( $args ) );

	// The number of attachments.
	$num_attachments = count($attachments);

	// The index of the last attachment (we decrement because numeric arrays start with 0).
	$last = $num_attachments - 1;

	// Counts which array key we're on as we loop.  We'll use this value to grab the next/prev post in our numeric array.
	$i=0;
 
 	// For each attachment...
	foreach ( $attachments as $k => $attachment ) {

		// If we hit the current post ...
		if ( $attachment->ID == $post->ID ) {
			
			// If we want the previous post ...
			if( $next_prev == 'prev' ){
				
				// Decrement the counter.
				$i--;

				// If there is a previous post, grab it.
				if( isset($attachments[$i]) ) {
					$adjacent_image = $attachments[$i];
					$icon = 'icon-chevron-left';
					$title_attr = 'Previous Image';
				
				// If there is not a previous post, it's because we're on the first post, so grab the final image
				} else {
					$adjacent_image = $attachments["$last"];
					$icon = 'icon-chevron-sign-left';
					$title_attr = 'Final Image';
				}

				// Exit the foreach loop.
				break;
			
			// If we want the next post ...
			} else {

				// Increment the counter.
				$i++;

				// If there is a next post, grab it.
				if( isset( $attachments[$i] ) ) {
					$adjacent_image = $attachments[$i];
					$icon = 'icon-chevron-right';
					$title_attr = 'Next Image';

				// If there is not a next post, it's because we're on the last post, so grab the first post.
				} else {
					$adjacent_image = $attachments[0];
					$icon = 'icon-chevron-sign-right';
					$title_attr = 'First Image';
				}

				// Exit the foreach loop.
				break;
			}
		}
	
		$i++;
	} // End foreach post.

	// Grab the href for the adjacent post
	$adjacent_image_id = absint( $adjacent_image->ID );
	$href = esc_url( get_attachment_link( $adjacent_image_id ) );

	// Complete the output by adding a FontAwesome
	$out = "<a title='$title_attr' href='$href'><i class='$icon'></i> </a>";
	return $out;
}