return DB::select('*')->from('users')->where("id",'=',$id);

<?php
    class Db {
        private static $db;
        private static $host = 'localhost';
        private static $dbname = 'test';
        private static $charset = 'utf8';
        private static $username = 'root';
        private static $password = '';

        private $select;
        private $from;
        private $id;

        static function connect() {
            if(empty(self::$db)) {
                 self::$db = new PDO("mysql:host=".self::$host.";dbname=".self::$dbname.";charset=".self::$charset,
                                        self::$username, self::$password);
            }

            return self::$db;
        }


        public function select($mark) {
            $this->select = 'SELECT ' . $mark;
            return $this;
        }

        public function from($table) {
            $this->from = ' FROM ' . $table;
            return $this;
        }

        public function where($id) {
            $this->where = ' WHERE id = ' . $id;
            return $this;
        }

    }

    class User extends Db {
        public function __toString() {
            return $this->viewUser();
        }

        public function viewUser() {
            return Db::select('*')->from('users')->where('1');
        }
    }

    $user = new User;
    echo $user->viewUser();

print_r((array)$user->viewUser());

и получился какой-то бред

Array ( [Dbselect] => SELECT * [Dbfrom] => FROM users [Dbid] => [where] => WHERE id = 1 )

$user = new User;
$str = '';
foreach((array)$user->viewUser() as $key) {
    $str = $str . $key;
}
echo $str;