apply_filters( 'post_gallery', string $output, array $attr, int $instance )

2851	 * @see gallery_shortcode()2852	 *2853	 * @param string $output   The gallery output. Default empty.2854	 * @param array  $attr     Attributes of the gallery shortcode.2855	 * @param int    $instance Unique numeric ID of this gallery shortcode instance.2856	 */2857	$output = apply_filters( 'post_gallery', '', $attr, $instance );2858 2859	if ( ! empty( $output ) ) {2860		return $output;2861	}2862 2863	$html5 = current_theme_supports( 'html5', 'gallery' );