How to retrieve featured image URL in WordPress

If (for some reason) you have the need to get the featured image (or post thumbnail) URL, you can do it with wp_get_attachment_image_src() function, for example like this:

<?php if (has_post_thumbnail( $post->ID ) ): ?>
<?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID, 'full' ), 'single-post-thumbnail' ); ?>
<div id="custom-bg" style="background-image: url('<?php echo $image[0]; ?>')">
<?php endif; ?>
</div>

<?php if (has_post_thumbnail( $post->ID ) ): ?>

<?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID, 'full' ), 'single-post-thumbnail' ); ?>

<?php endif; ?>

</div>