about flash function

[wfsdaj]

flash('a', 'b');
echo flash('a');  // output empty
echo flash('b');  // output 'a'

so, 'b' is a key, 'a' is a message?

// leafs/session/src/functions.php

if (!function_exists('flash') && class_exists('Leaf\App')) {
    /**
     * Return flash data/object or set flash data
     *
     * @param string|null $key — The flash data to set/get
     * @param mixed $key — The data to set
     */
    function flash($key = null, $value = null)
    {
        if (!$key && !$value) {
            if (!(\Leaf\Config::get("flash.instance"))) {
                \Leaf\Config::set("flash.instance", new \Leaf\Flash());
            }

            return \Leaf\Config::get("flash.instance");
        }

        if (!$value && is_string($key)) {
            return \Leaf\Flash::display($key);
        }

        return \Leaf\Flash::set($key, $value);
    }
}