[WordPress] Buffering WordPress Content with PHP

00-the-content.php

<?php

add_filter( 'the_content', 'my_content_filter' );
function my_content_filter( $content ) {  
  
  $content .= 'Thanks for wasting your time reading this post!'; 
  return $content;
}

01-buffering-wordpress-content.php

<?php

public function init() {

  // Grab the ID of the page for which we want to apply the buffered content.
	$page_id = $this->get_the_page_ID();
	if ( get_the_ID() !== $page_id ) {
		return;
	}

	add_filter( 'the_content', array( $this, 'the_content' ) );
}

public function the_content( $content ) {

	// Grab the users associated with this installation.
	$users = $this->get_users();

	// We capture the partial into a buffer so we can append it to the existing content.
	ob_start();
	include_once 'directory-partial.php';
	$content .= ob_get_clean();

	return $content;
}