How to set featured image to custom post from outside programmatically

Can’t this simply be done with media_sideload_image() ?

Seems pretty simple. Only catch is if you aren’t on admin area, you must include some libraries from within WordPress includes:

// only need these if performing outside of admin environment
require_once(ABSPATH . 'wp-admin/includes/media.php');
require_once(ABSPATH . 'wp-admin/includes/file.php');
require_once(ABSPATH . 'wp-admin/includes/image.php');

// example image
$image="http://example.com/logo.png";

// magic sideload image returns an HTML image, not an ID
$media = media_sideload_image($image, $post_id);

// therefore we must find it so we can set it as featured ID
if(!empty($media) && !is_wp_error($media)){
    $args = array(
        'post_type' => 'attachment',
        'posts_per_page' => -1,
        'post_status' => 'any',
        'post_parent' => $post_id
    );

    // reference new image to set as featured
    $attachments = get_posts($args);

    if(isset($attachments) && is_array($attachments)){
        foreach($attachments as $attachment){
            // grab source of full size images (so no 300x150 nonsense in path)
            $image = wp_get_attachment_image_src($attachment->ID, 'full');
            // determine if in the $media image we created, the string of the URL exists
            if(strpos($media, $image[0]) !== false){
                // if so, we found our image. set it as thumbnail
                set_post_thumbnail($post_id, $attachment->ID);
                // only want one image
                break;
            }
        }
    }
}