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

2708	 * @see gallery_shortcode()2709	 *2710	 * @param string $output   The gallery output. Default empty.2711	 * @param array  $attr     Attributes of the gallery shortcode.2712	 * @param int    $instance Unique numeric ID of this gallery shortcode instance.2713	 */2714	$output = apply_filters( 'post_gallery', '', $attr, $instance );2715 2716	if ( ! empty( $output ) ) {2717		return $output;2718	}2719 2720	$html5 = current_theme_supports( 'html5', 'gallery' );