system/Session/Session.php
public function getFlashdata(string $key = null)
	{
		if (isset($key))
		{
			return (isset($_SESSION['__ci_vars'], $_SESSION['__ci_vars'][$key], $_SESSION[$key]) &&
					! is_int($_SESSION['__ci_vars'][$key])) ? $_SESSION[$key] : null;
		}

		$flashdata = [];

		if (! empty($_SESSION['__ci_vars']))
		{
			foreach ($_SESSION['__ci_vars'] as $key => &$value)
			{
				is_int($value) || $flashdata[$key] = $_SESSION[$key];
			}
		}

		return $flashdata;
	}
public function getFlashdata(string $key = null)
	{
		if (isset($key))
		{
			return (isset($_SESSION['__ci_vars'], $_SESSION['__ci_vars'][$key], $_SESSION[$key]) &&
					! is_int($_SESSION['__ci_vars'][$key])) ? $_SESSION[$key] : null;
		}

		$flashdata = [];

		if (! empty($_SESSION['__ci_vars']))
		{
			foreach ($_SESSION['__ci_vars'] as $key => &$value)
			{
				is_int($value) || $flashdata[$key] = $_SESSION[$key];
			}
		}

		return $flashdata;
	}
Retrieve one or more items of flash data from the session.

If the item key is null, return all flashdata.

@param  string $key Property identifier
@return array|null	The requested property value, or an associative array  of them