This filter is very useful. One of the common problems for developers is how to include templates in plugins they develop.

The filter is applied immediately after wordpress locates the appropriate template in the active child/parent theme using the wp hierarchy.

Be careful to define when you want to modify the template path. In the below example, the code checks to see if the current page is the single view of our custom post type cpt.

Simple enough example to get started with!

add_filter('template_include', 'custom_function');


function custom_function($template){
    
    //change a single post template...
    
    if( is_singular('cpt')  ){
         $template= 'path/to/another/template_file';
    }
      
    
    return $template;

}

add_filter('template_include', 'custom_function');


function custom_function($template){
    
    /*
    *     This example is a little more advanced. 
    *    It will check to see if $template contains our post-type in the path.
    *    If it does, the theme contains a high level template e.g. single-cpt.php
    *    If not we look in the plugin parent folder for the file. e.g. single-cpt.php
    */
    
  
    //check to see if the post type is in the filename (high priority file)
    //return template if it is!
    
    global $post;
   
    if( strpos($template, 'single-'.$post->post_type.'php' ) !== false && strpos($template, 'archive-'.$post->post_type.'php' ) !== false ){
        return $template;
    }
    
    $plugin_path = 'var/etc/wp-content/plugins/plugin'; //include own logic here...
    
    if( is_singular($post->post_type) && file_exists($plugin_path.'/single-'.$post->post_type.'.php')   ){
         $template= $plugin_path.'/single-'.$post->post_type.'.php';
    } elseif ( is_archive($post->post_type) && file_exists($plugin_path.'/archive-'.$post->post_type.'.php') ) {
         $template= $plugin_path.'/archive-'.$post->post_type.'.php';
    } 
      
    return $template;
}