test.php

  public static function search_users_parameters() {
        return new external_function_parameters(
            array(
                'name' => new external_value(PARAM_RAW, 'users name', VALUE_OPTIONAL),
                'page' => new external_value(PARAM_RAW, 'pagination', VALUE_OPTIONAL),
                'faculty' => new external_value(PARAM_RAW, 'Faculty id', VALUE_OPTIONAL),
                'course' => new external_value(PARAM_INT, 'Course id', VALUE_OPTIONAL)
            )
        );
    }

    private function _filterByFaculcy(){
        return true;        
    }

    public static function search_users($name, $page, $faculty, $course) {
        
        global $CFG, $DB;
        
        //Params validation
        $params = self::validate_parameters(
            self::search_users_parameters(),
            array(
                'name' => $name, 
                'page' => $page, 
                'faculty' => $faculty,
                'course' => $course
            )
        );

        $where = '';
        $join = '';
        $faculties = self::getFaculcy();

        if(strlen($name) > 0) {
            $where .= " AND concat(u.firstname, ' ', u.lastname) LIKE '%" . $name . "%' ";
        }

        if(strlen($faculty) > 0){
            if(isset($faculties[$faculty])){
                $join .= "LEFT JOIN {cohort_members} mb ON mb.userid = u.id 
                          LEFT JOIN {cohort} c ON c.id = mb.cohortid";
                $where .= " AND c.name LIKE '".$faculty."_%' ";
            } else {
                echo '{"error":"Faculcy with those id not registered"}'; exit;
            }
        }

        if((int)$course > 0 && (int)$course <= 9){
            if(isset($faculties[$faculty])){
                if(strlen($faculty) == 0){
                    $join .= "LEFT JOIN {cohort_members} mb ON mb.userid = u.id 
                              LEFT JOIN {cohort} c ON c.id = mb.cohortid";
                }
                $where .= " AND c.name LIKE '%_".$course."' ";
            } else {
                echo '{"error":"Wrong course value"}'; exit;
            }
        }

        $page = $page > 0 ? ($page -1) : 0;

        $pagination = 20 * $page + 20;

        $sql = "SELECT 
                    u.id,
                    u.firstname,
                    u.lastname,
                    u.picture 
                    FROM {user} u $join 
                        WHERE 1 $where 
                            LIMIT $pagination, 20";

        $users = $DB->get_records_sql($sql);

        $res = array();

        foreach ($users as $k => $v) {
            $res[] = self::getUserImage($v);
        }

        echo json_encode($res, JSON_UNESCAPED_UNICODE); exit;
        // return $res;
    }