grandchild-functions.php

<?php
/*
Plugin Name: UTC Institute
Description: Sample grandchild theme built as a plugin.
Author: Web Dev
Author URI: http://www.your-website.com
Version: 1.0
*/
 

require_once( plugin_dir_path( __FILE__ ) . 'template-class.php' );
add_action( 'plugins_loaded', array( 'Page_Template_Plugin', 'get_instance' ) );

// Adds our new file with styles
function grandchild_add_scripts_styles() {
	wp_register_script( 'grandchild-script', plugins_url( 'grandchild-scripts.js', __FILE__ ), array( 'jquery' ), '1.0' );
	wp_enqueue_script( 'grandchild-script' );
	
	wp_register_style( 'grandchild-style', plugins_url( 'grandchild-styles.css', __FILE__ ), array(), '1.0' );
	wp_enqueue_style( 'grandchild-style' );
}
add_action( 'wp_enqueue_scripts', 'grandchild_add_scripts_styles', 99999999 );

// Search for templates in plugin 'templates' dir, and load if exists
function grandchild_template_include( $template ) {
	if ( file_exists( untrailingslashit( plugin_dir_path( __FILE__ ) ) . '/templates/' . basename( $template ) ) )
		$template = untrailingslashit( plugin_dir_path( __FILE__ ) ) . '/templates/' . basename( $template );

	return $template;
}
add_filter( 'template_include', 'grandchild_template_include', 11 );

if ( function_exists('register_sidebar') ) {
	register_sidebar( array(
		'name'         => __( 'Header - Image' ),
		'id'           => 'header-image',
		'description'  => __( 'Widgets in this area will be shown in the header.' ),
		'before_title' => '<h1>',
		'after_title'  => '</h1>',
	) );
	
	register_sidebar( array(
		'name'         => __( 'Footer - Search' ),
		'id'           => 'footer-search',
		'description'  => __( 'Widgets in this area will be shown on the right hand side of the footer.' ),
		'before_title' => '<h1>',
		'after_title'  => '</h1>',
	) );
}

// We are hooking to the 'load_custom_header' filter 
// and return a value of 'true' meaning that we want to 
// load a custom header
add_filter( 'load_custom_header', '__return_true', 99 );

add_filter( 'get_custom_header', function( $header ) {
    return plugin_dir_path( __FILE__ ).'templates/header.php';
}, 99 );

add_filter( 'load_custom_footer', '__return_true', 99 );

add_filter( 'get_custom_footer', function( $footer ) {
    return plugin_dir_path( __FILE__ ).'templates/footer.php';
}, 99 );

add_filter('template_include', 'my_plugin_templates');
function my_plugin_templates( $template ) {
    $post_types = array('post');

    if (is_singular($post_types)) {
        $template = plugin_dir_path( __FILE__ ).'templates/single.php';
    }
	
    return $template;
}