attach media that was uploaded in post as thumbnails

attach_to_post.php

<?php

add_action('init', 'update_business_thumbnails');

function update_business_thumbnails()
{
    if (!isset($_GET['update_thumbnails'])) return ;

    $args = [
        'post_type'      => 'attachment',
        'posts_per_page' => -1,
        'post_status'    => ['all'],
        'fields'         => 'ids'
    ];

    $mediaIds = (new WP_Query($args))->posts;

    foreach ($mediaIds as $fileId) {
        print_r($fileId. '\n');
        $postId = wp_get_post_parent_id( $fileId );
        if (!$postId) {
            return;
        }
        if (has_post_thumbnail($postId)) {
            return;
        }
        $updated = update_post_meta( $postId, '_thumbnail_id', $fileId );
        if ( $updated ) {
            wp_cache_set( 'last_changed', microtime(), 'posts' );
            dump('updated\n');
        } else {
            dump('couldnt update'. (get_post($postId))->post_title . '\n');
        }
    }

    die('Done');

}

This grabs to parent post of the attachment, which is the post it was uploaded into and makes it a thumbnail of the post.