shortcodes.php

<?php
/**
 * YouTube Gallery
 *
 * @package      CoreFunctionality
 * @author       Bill Erickson
 * @since        1.0.0
 * @license      GPL-2.0+
**/

/**
 * Youtube Gallery
 */
function ea_youtube_gallery_shortcode() {

	$output = '<div class="youtube-gallery">';

	// Video Player
	$output .= '<div class="youtube-gallery--player">';
	$output .= '<h4>Video goes here</h4>';
	$output .= '</div>';

	// Video Listing
	$output .= '<div class="youtube-gallery--listing">';
	for( $i = 0; $i < 24; $i++ ) {
		$class = 0 == $i ? 'item active' : 'item';
		$output .= '<a href="#main-content" class="' . $class . '">';
		$output .= '<span class="title">thumbnail goes here</span>';
		$output .= '</a>';
	}
	$output .= '</div>';

	$output .= '</div>';

	return $output;
}
add_shortcode( 'ea_youtube_gallery', 'ea_youtube_gallery_shortcode' );