Retrieves the post thumbnail.

When a theme adds ‘post-thumbnail’ support, a special ‘post-thumbnail’ image size
is registered, which differs from the ‘thumbnail’ image size managed via the
Settings > Media screen.

When using the_post_thumbnail() or related functions, the ‘post-thumbnail’ image
size is used by default, though a different size can be specified instead as needed.

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();
		}

		// Get the 'loading' attribute value to use as default, taking precedence over the default from
		// `wp_get_attachment_image()`.
		$loading = wp_get_loading_attr_default( 'the_post_thumbnail' );

		// Add the default to the given attributes unless they already include a 'loading' directive.
		if ( empty( $attr ) ) {
			$attr = array( 'loading' => $loading );
		} elseif ( is_array( $attr ) && ! array_key_exists( 'loading', $attr ) ) {
			$attr['loading'] = $loading;
		} elseif ( is_string( $attr ) && ! preg_match( '/(^|&)loading=/', $attr ) ) {
			$attr .= '&loading=' . $loading;
		}

		$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, or 0 if there isn't one.
	 * @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|array $attr              Query string or array of attributes.
	 */
	return apply_filters( 'post_thumbnail_html', $html, $post->ID, $post_thumbnail_id, $size, $attr );
}