WordPress | Thumbnail images in search results

wp-meta-thumbnail.php

<?php

/**
 * Thumbnail images in search results
 *
 * @see https://support.google.com/customsearch/answer/1626955
 */
add_action( 'wp_head', function() {
    if ( is_single() && '' !== ($thumbnail_id = get_post_thumbnail_id()) && false !== ($image = wp_get_attachment_image_src( $thumbnail_id ))) {
        echo "<meta name=\"thumbnail\" content=\"{$image[0]}\">";
    }
} );