Method has

/**
 * Check whether data exists in the session store
 *
 * @param   string  $name  Name of variable
 *
 * @return  boolean  True if the variable exists
 *
 * @since   1.5
 */
public function has($name)
{
    // Handle B/C by checking if a namespace was passed to the method, will be removed at 5.0
    if (\func_num_args() > 1) {
        $args = \func_get_args();
        if (!empty($args[1])) {
            @trigger_error('Passing a namespace as a parameter to ' . __METHOD__ . '() is deprecated. ' . 'The namespace should be prepended to the name instead.', E_USER_DEPRECATED);
            $name = $args[1] . '.' . $name;
        }
    }
    if (parent::has($name)) {
        return true;
    }
    /*
     * B/C for retrieving sessions that originated in Joomla 3.
     * A namespace before Joomla 4 has a prefix of 2 underscores (__).
     * This is no longer the case in Joomla 4 and will be converted
     * when saving new values in `self::set()`
     */
    if (strpos($name, '.') !== false && parent::has('__' . $name)) {
        return true;
    }
    // More b/c for retrieving sessions that originated in Joomla 3. This will be removed in Joomla 5
    // as no sessions should have this format anymore!
    return parent::has('__default.' . $name);
}