Append Series List to WordPress Posts

append-series-list.php

<?php
add_filter( 'the_content', 'uamv_series_list', 2 );

function uamv_series_list( $content ) {

	// associative array with seriesName => array of post IDs
	$series = array(
		'#MissionText' => array( 5671, 5691, 5741, 5814 ),
		'Prayer' => array( 171, 220, 235, 237, 239, 241, 3890 ),
	);
	
	foreach ( $series as $list => $ids ) {
	
		// we'll display the series list only on single posts
		if ( is_single( $ids ) ) {

			$content .= '<p style="font-size:75%;text-align:right;"><strong>Other posts in this <em>' . esc_html( $list ) . '</em> series:</strong><br />';

			// insert link to each post in the series
			foreach ( $ids as $key => $id ) {
				// prepend a separator to each post - after the first
				$content .= $key > 0 ? ' • ' : '';
				$content .= '<a href="' . get_permalink( $id ) . '">' . get_the_title( $id ) . '</a>';
			}

			$content .= '</p>';

		}

	}

	return $content;

}