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

2734	 * @see gallery_shortcode()2735	 *2736	 * @param string $output   The gallery output. Default empty.2737	 * @param array  $attr     Attributes of the gallery shortcode.2738	 * @param int    $instance Unique numeric ID of this gallery shortcode instance.2739	 */2740	$output = apply_filters( 'post_gallery', '', $attr, $instance );2741 2742	if ( ! empty( $output ) ) {2743		return $output;2744	}2745 2746	$html5 = current_theme_supports( 'html5', 'gallery' );