The Invisible Preview Page - Filters (Updated)

preview-updated.php

<?php

if ( isset( $_GET['my_preview_page'] ) ){

  add_filter('the_title', 'my_preview_title' );
  function my_preview_title(){
    return "My Preview Page";
  }

  add_filter('the_content', 'my_preview_content' );
  add_filter('get_the_excerpt', 'my_preview_content' );
  function my_preview_content(){
    return "My Preview Content Here";
  }
  
  add_filter('template_include', 'my_preview_template' );
  function my_preview_template(){
    return locate_template( array( 'page.php', 'single.php', 'index.php' ) );
  }
}