Get the thumbnail source URL in WordPress

gistfile1.php

function wp_get_thumb_src($post = false, $size = 'thumbnail') {
  if ( ! $post ) {
    global $post;
  }
  
  $thumb = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), $size );
  $url = $thumb['0'];
  
  return $url;
}