framework/web/Session.php
public function offsetGet($offset)
    {
        $this->open();

        return isset($_SESSION[$offset]) ? $_SESSION[$offset] : null;
    }
public function offsetGet($offset)
    {
        $this->open();

        return isset($_SESSION[$offset]) ? $_SESSION[$offset] : null;
    }
This method is required by the interface [[\ArrayAccess]].
@param int $offset the offset to retrieve element.
@return mixed the element at the offset, null if no element is found at the offset