Add Friends Of ACL to Elgg access levels

start.php

<?php

/**
 * Define a new access level to allow owners to save content visible to those who added them as a friend,
 * as oppose to Friends access level, which shows content to users that have been added as friends by the owner
 *
 * Note that Elgg friendships are not reciprocal by default
 * 
 * @author Ismayil Khayredinov <[email protected]>
 * @copyright 2016 (c) Ismayil Khayredinov
 * @license GNU GPL 2.0
 */
define('ACCESS_FRIENDS_OF', -2); // make sure this is not used by other plugins

elgg_register_event_handler('init', 'system', function() {
  elgg_register_plugin_hook_handler('get_sql', 'access', 'friends_of_acl_get_access_sql_clauses');
  elgg_register_plugin_hook_handler('access:collections:write', 'user', 'friends_of_acl_access_collections_write');
});


/**
 * Populates access WHERE sql clauses
 *
 * @param string $hook   "get_sql"
 * @param string $type   "access"
 * @param array  $return Clauses
 * @param array  $params Hook params
 * @return array
 */
function friends_of_acl_get_access_sql_clauses($hook, $type, $return, $params) {

	$ignore_access = elgg_extract('ignore_access', $params);
	if ($ignore_access) {
		return;
	}
	$user_guid = elgg_extract('user_guid', $params);
	if (!$user_guid) {
		return;
	}

	$prefix = elgg_get_config('dbprefix');
	$table_alias = $params['table_alias'] ? $params['table_alias'] . '.' : '';
	$access_column = $params['access_column'];
	$owner_guid_column = $params['owner_guid_column'];

	$id = ACCESS_FRIENDS_OF;

	$return['ors'][] = "{$table_alias}{$access_column} = {$id}
			AND {$table_alias}{$owner_guid_column} IN (
				SELECT guid_two FROM {$prefix}entity_relationships
				WHERE relationship='friend' AND guid_one = {$user_guid}
			)";
				
	return $return;
}

/**
 * Populates write access array with relationship ACLs
 *
 * @param string $hook   "access:collections:write"
 * @param string $type   "user"
 * @param array  $return ACL IDs
 * @param array  $params Hook params
 * @return array
 */
function friends_of_acl_access_collections_write($hook, $type, $return, $params) {

	$page_owner = elgg_get_page_owner_entity();
	if ($page_owner instanceof ElggGroup) {
		return;
	}

	$return[ACCESS_FRIENDS_OF] = elgg_echo('access:friends_of');

	return $return;
}