Delete Extra Plugin Tables When a Site Is Deleted in WordPress Multisite

delete-plugin-tables.php

<?php
/**
 * Add any custom tables that this plugin creates for an individual site to the list
 * 	of tables that get deleted when a site is deleted.
 * @see https://core.trac.wordpress.org/browser/tags/4.2.2/src/wp-admin/includes/ms.php#L116
 */
add_filter( 'wpmu_drop_tables', 'delete_my_plugin_tables', 10, 2 );
function delete_my_plugin_tables( $tables=array(), $blog_id=null ) {
	/**
	 * Make sure the blog ID parameter was sent, so we don't
	 * 	accidentally delete tables for the wrong blog
	 */
	if ( empty( $blog_id ) || 1 == $blog_id || $blog_id != $GLOBALS['blog_id'] )
		return $tables;
	
	/**
	 * Assume our plugin added three new tables called "plugin_table_1", "plugin_table_2" and "plugin_table_3"
	 * 	to each site on which it's active
	 */
	global $wpdb;
	$blog_prefix = $wpdb->get_blog_prefix( $blog_id );
	$base_prefix = $wpdb->base_prefix;
	$plugin_tables = array( 'plugin_table_1', 'plugin_table_2', 'plugin_table_3' );
	/**
	 * Since the $wpdb->tables() call in the wpmu_delete_blog() function is called without the 
	 * 	$prefix parameter, the list of tables sent through this filter will all be prefixed 
	 * 	with the appropriate blog prefix before it gets to this filter. We need to prefix 
	 * 	our list of tables, as well, before sending it back.
	 */
	foreach ( $plugin_tables as $k => $table ) {
		$tables[$table] = $blog_prefix . $table;
	}
	
	return $tables;
}