src/Bundle/Common/Session/SessionBag.php
public function has(string $key): bool
    {
        return isset($_SESSION) ? array_key_exists($key, $_SESSION) : false;
    }
public function has(string $key): bool
    {
        return isset($_SESSION) ? array_key_exists($key, $_SESSION) : false;
    }
Get true if the SESSION parameter is defined.

@param string $key
@return bool true