WordPress plugin to make plugin's own Page Templates available to the website.

class-plugin-template-include.php

<?php

class Plugin_Templates_Loader {
  
    /**
     * Templates folder inside the plugin
     */
    private $templates_dir;

    /**
     * Templates to be merged with WP
     */
    private $templates;

    /**
     * Filtering and loading templates
     */
    public function __construct ( ) {
        $this->template_dir = plugin_dir_path(__FILE__) . '../templates/';

        $this->templates = $this->load_plugin_templates();

        add_filter('theme_page_templates', array($this, 'register_templates_filter'));
        add_filter('template_include', array($this, 'add_template_filter' ));
    }

    /**
     * Loading templates from the templates folder inside the plugin
     */
    private function load_plugin_templates ( ) {

        $template_dir = $this->template_dir;

        // Reads all templates from the folder
        if (is_dir($template_dir)) {
            if ($dh = opendir($template_dir)) {
                while (($file = readdir($dh)) !== false) {

                    $full_path = $template_dir . $file;

                    if (filetype($full_path) == 'dir') {
                        continue;
                    }

                    // Gets Template Name from the file
                    $filedata = get_file_data($full_path, array(
                        'Template Name' => 'Template Name',
                    ));

                    $template_name = $filedata['Template Name'];

                    $templates[$full_path] = $template_name;

                }
                closedir($dh);
            }
      }		

      return $templates;
    }

    /**
     * theme_page_templates Filter callback
     *
     * Merges plugins' template with theme's, making them available for the user
     * 
     * @param array $theme_templates
     * @return array $theme_templates
     */
    public function register_plugin_templates ( $theme_templates ) {

        // Merging the WP templates with this plugin's active templates
        $theme_templates = array_merge($theme_templates, $this->templates);
        
        return $theme_templates;
    }

    /**
     * template_include Filter callback
     * 
     * Include plugin's template if there's one chosen for the rendering page 
     *
     * @param string $template path
     * @return string $template path
     */
     public function add_template_filter ( $template ) {
        
        $user_selected_template = get_page_template_slug($post->ID);

        // We need to check if the selected template
        // is inside the plugin folder
        $file_name = pathinfo($user_selected_template, PATHINFO_BASENAME);
        $template_dir = $this->template_dir;

        if (file_exists($template_dir . $file_name)) {
            $is_plugin = true;
        }

        // If selected template is not empty, it's not the Default Template
        // AND if it's a plugin template, we replace the normal flow to
        // include the selected template
        if ( $user_selected_template != '' AND $is_plugin ) {
            $template = $user_selected_template;
        }       
    
        return $template;
    }   
    
}