Helper function for getting the featured image URL attached to a post (WordPress)

functions.php

<?php

function get_thumbnail_url($post_id = null, $size = 'medium')
{
    global $post;

    if (is_string($post_id)) {
        $size    = $post_id;
        $post_id = $post->ID;
    }

    if (is_null($post_id)) {
        $post_id = $post->ID;
    }

    $thumbnailUrl = wp_get_attachment_image_src(get_post_thumbnail_id($post_id), $size, false, '');

    if (is_array($thumbnailUrl)) {
        return $thumbnailUrl[0];
    }

    return '';
}