get_the_post_thumbnail( int|WP_Post $post = null, string|array $size = 'post-thumbnail', string|array $attr = '' )

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 WP-2.9.0
	 * @since WP-4.9.0 Added the `$post_id` parameter.
	 *
	 * @param string|array $size    The post thumbnail size. Image size or array of width and height
	 *                              values (in that order). Default 'post-thumbnail'.
	 * @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 WP-2.9.0
		 *
		 * @param int          $post_id           The post ID.
		 * @param string       $post_thumbnail_id The post thumbnail ID.
		 * @param string|array $size              The post thumbnail size. Image size or array of width
		 *                                        and height values (in that order). Default 'post-thumbnail'.
		 */
		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 WP-2.9.0
		 *
		 * @param int          $post_id           The post ID.
		 * @param string       $post_thumbnail_id The post thumbnail ID.
		 * @param string|array $size              The post thumbnail size. Image size or array of width
		 *                                        and height values (in that order). Default 'post-thumbnail'.
		 */
		do_action( 'end_fetch_post_thumbnail_html', $post->ID, $post_thumbnail_id, $size );

	} else {
		$html = '';
	}
	/**
	 * Filters the post thumbnail HTML.
	 *
	 * @since WP-2.9.0
	 *
	 * @param string       $html              The post thumbnail HTML.
	 * @param int          $post_id           The post ID.
	 * @param string       $post_thumbnail_id The post thumbnail ID.
	 * @param string|array $size              The post thumbnail size. Image size or array of width and height
	 *                                        values (in that order). Default 'post-thumbnail'.
	 * @param string       $attr              Query string of attributes.
	 */
	return apply_filters( 'post_thumbnail_html', $html, $post->ID, $post_thumbnail_id, $size, $attr );
}