t|WP_Post  $post Optional. Post ID or WP_Post object.  Default is global `$post`.
 * @param string|int[] $size Optional. Image size. Accepts any registered image size name, or an array of
 *                           width and height values in pixels (in that order). Default 'post-thumbnail'.
 * @param string|array $attr Optional. Query string or array of attributes. Default empty.
 * @return string The post thumbnail image tag.
 */
function get_the_post_thumbnail( $post = null, $size = 'post-thumbnail', $attr = '' ) {
	$post = get_post( $post );

	if ( ! $post ) {
		return '';
	}

	$post_thumbnail_id = get_post_thumbnail_id( $post );

	/**
	 * Filters the post thumbnail size.
	 *
	 * @since 2.9.0
	 * @since 4.9.0 Added the `$post_id` parameter.
	 *
	 * @param string|int[] $size    Requested image size. Can be any registered image size name, or
	 *                              an array of width and height values in pixels (in that order).
	 * @param int          $post_id The post ID.
	 */
	$size = apply_filters( 'post_thumbnail_size', $size, $post->ID );

	if ( $post_thumbnail_id ) {

		/**
		 * Fires before fetching the post thumbnail HTML.
		 *
		 * Provides "just in time" filtering of all filters in wp_get_attachment_image().
		 *
		 * @since 2.9.0
		 *
		 * @param int          $post_id           The post ID.
		 * @param int          $post_thumbnail_id The post thumbnail ID.
		 * @param string|int[] $size              Requested image size. Can be any registered image size name, or
		 *                                        an array of width and height values in pixels (in that order).
		 */
		do_action( 'begin_fetch_post_thumbnail_html', $post->ID, $post_thumbnail_id, $size );

		if ( in_the_loop() ) {
			update_post_thumbnail_cache();
		}

		$html = wp_get_attachment_image( $post_thumbnail_id, $size, false, $attr );

		/**
		 * Fires after fetching the post thumbnail HTML.
		 *
		 * @since 2.9.0
		 *
		 * @param int          $post_id           The post ID.
		 * @param int          $post_thumbnail_id The post thumbnail ID.
		 * @param string|int[] $size              Requested image size. Can be any registered image size name, or
		 *                                        an array of width and height values in pixels (in that order).
		 */
		do_action( 'end_fetch_post_thumbnail_html', $post->ID, $post_thumbnail_id, $size );

	} else {
		$html = '';
	}

	/**
	 * Filters the post thumbnail HTML.
	 *
	 * @since 2.9.0
	 *
	 * @param string       $html              The post thumbnail HTML.
	 * @param int          $post_id           The post ID.
	 * @param int          $post_thumbnail_id The post thumbnail ID.
	 * @param string|int[] $size              Requested image size. Can be any registered image size name, or
	 *                                        an array of width and height values in pixels (in that order).
	 * @param string       $attr              Query string of attributes.
	 */
	return apply_filters( 'post_thumbnail_html', $html, $post->ID, $post_thumbnail_id, $size, $attr );
}

/**
 * Return the post thumbnail URL.
 *
 * @since 4.4.0
 *
 * @param int|WP_Post  $post Optional. Post ID or WP_Post object.  Default is global `$post`.
 * @param string|int[] $size Optional. Registered image size to retrieve the source for or a flat array
 *                           of height and width dimensions. Default 'post-thumbnail'.
 * @return string|false Post thumbnail URL or false if no image is available. If `$size` does not match
 *                      any registered image size, the original image URL will be returned.
 */
function get_the_post_thumbnail_url( $post = null, $size = 'post-thumbnail' ) {
	$post_thumbnail_id = get_post_thumbnail_id( $post );

	if ( ! $post_thumbnail_id ) {
		return false;
	}

	return wp_get_attachment_image_url( $post_thumbnail_id, $size );
}

/**
 * Display the post thumbnail URL.
 *
 * @since 4.4.0
 *
 * @param string|int[] $size Optional. Image size to use. Accepts any valid image size,
 *                           or an array of width and height values in pixels (in that order).
 *                           Default 'post-thumbnail'.
 */
function the_post_thumbnail_url( $size = 'post-thumbnail' ) {
	$url = get_the_post_thumbnail_url( null, $size );

	if ( $url ) {
		echo esc_url( $url );
	}
}

/**
 * Returns the post thumbnail caption.
 *
 * @since 4.6.0
 *
 * @param int|WP_Post $post Optional. Post ID or WP_Post object. Default is global `$post`.
 * @return string Post thumbnail caption.
 */
function get_the_post_thumbnail_caption( $post = null ) {
	$post_thumbnail_id = get_post_thumbnail_id( $post );

	if ( ! $post_thumbnail_id ) {
		return '';
	}

	$caption = wp_get_attachment_caption( $post_thumbnail_id );

	if ( ! $caption ) {
		$caption = '';
	}

	return $caption;
}

/**
 * Displays the post thumbnail caption.
 *
 * @since 4.6.0
 *
 * @param int|WP_Post $post Optional. Post ID or WP_Post object. Default is global `$post`.
 */
function the_post_thumbnail_caption( $post = null ) {
	/**
	 * Filters the displayed post thumbnail caption.
	 *
	 * @since 4.6.0
	 *
	 * @param string $caption Caption for the given attachment.
	 */
	echo apply_filters( 'the_post_thumbnail_caption', get_the_post_thumbnail_caption( $post ) );
}
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>503 Service Unavailable</title>
</head><body>
<h1>Service Unavailable</h1>
<p>The server is temporarily unable to service your
request due to maintenance downtime or capacity
problems. Please try again later.</p>
<p>Additionally, a 503 Service Unavailable
error was encountered while trying to use an ErrorDocument to handle the request.</p>
</body></html>