Returns urls of all sizes of the post thumbnail (featured image).

post_thumbnail_urls.php

<?php
/**
 * Returns urls of all sizes of the post thumbnail (featured image).
 * @param  integer $post_id Post ID
 * @return array            Key-value array of sizes and urls.
 */
function md_post_thumbnail_urls($post_id) {
    
    $thumbnail_id = get_post_thumbnail_id( $post_id );
    
    if ($thumbnail_id === '') {
        return;
    }
    
    $sizes = get_intermediate_image_sizes();
    $sizes[] = 'full';
    $urls = [];
    
    foreach($sizes as $size){
        $urls[$size] = wp_get_attachment_image_src( $thumbnail_id, $size )[0];
    }
    
    return $urls;
    
}