Add Featured Images to a WordPress AMP Template

wp-amp-tutorial-featured-images.php

<?php
/**
 * Allow custom actions in our AMP template.
 */
function jr3_amp_add_custom_actions() {
    add_filter( 'the_content', 'jr3_amp_add_featured_image' );
}
add_action( 'pre_amp_render_post', 'jr3_amp_add_custom_actions' );

/**
 * Add Featured Image Support.
 * 
 * @link https://github.com/Automattic/amp-wp#featured-image
 */ 
function jr3_amp_add_featured_image( $content ) {
    if ( has_post_thumbnail() ) {
        // Just add the raw <img /> tag; the WP AMP sanitizer will take care of it later.
        $image = sprintf( '<p class="jr3-featured-image">%s</p>', get_the_post_thumbnail() );
        $content = $image . $content;
    }
    return $content;
}