_transition_post_status( string $new_status, string $old_status, WP_Post $post )

function _transition_post_status( $new_status, $old_status, $post ) {
	global $wpdb;

	if ( $old_status != 'publish' && $new_status == 'publish' ) {
		// Reset GUID if transitioning to publish and it is empty.
		if ( '' == get_the_guid($post->ID) )
			$wpdb->update( $wpdb->posts, array( 'guid' => get_permalink( $post->ID ) ), array( 'ID' => $post->ID ) );

		/**
		 * Fires when a post's status is transitioned from private to published.
		 *
		 * @since WP-1.5.0
		 * @deprecated WP-2.3.0 Use 'private_to_publish' instead.
		 *
		 * @param int $post_id Post ID.
		 */
		do_action('private_to_published', $post->ID);
	}

	// If published posts changed clear the lastpostmodified cache.
	if ( 'publish' == $new_status || 'publish' == $old_status) {
		foreach ( array( 'server', 'gmt', 'blog' ) as $timezone ) {
			wp_cache_delete( "lastpostmodified:$timezone", 'timeinfo' );
			wp_cache_delete( "lastpostdate:$timezone", 'timeinfo' );
			wp_cache_delete( "lastpostdate:$timezone:{$post->post_type}", 'timeinfo' );
		}
	}

	if ( $new_status !== $old_status ) {
		wp_cache_delete( _count_posts_cache_key( $post->post_type ), 'counts' );
		wp_cache_delete( _count_posts_cache_key( $post->post_type, 'readable' ), 'counts' );
	}

	// Always clears the hook in case the post status bounced from future to draft.
	wp_clear_scheduled_hook('publish_future_post', array( $post->ID ) );
}