Function to set the featured image for a given post in WordPress

set-featured-image.php

<?php
/**
 * Function to set the featured image for a given post in WordPress
 *
 * @author Micah Wood <[email protected]>
 * @param int $post_id The ID of the post for which we are setting the featured image
 * @param int $attachment_id The ID of the attachment we want to use as the featured image
 */
function set_featured_image( $post_id, $attachment_id ){
     update_post_meta( $post_id, '_thumbnail_id', $attachment_id );
}