_stack = $stack; } public function push($value) { $this->_stack[] = $value; } public function pop() { return array_pop($this->_stack); } public function peek($offset = 1) { if (isset($this->_stack[count($this->_stack) - $offset])) { return $this->_stack[count($this->_stack) - $offset]; } else { return null; } } }