system/Session/Session.php
public function markAsFlashdata($key): bool
	{
		if (is_array($key))
		{
			for ($i = 0, $c = count($key); $i < $c; $i ++)
			{
				if (! isset($_SESSION[$key[$i]]))
				{
					return false;
				}
			}

			$new = array_fill_keys($key, 'new');

			$_SESSION['__ci_vars'] = isset($_SESSION['__ci_vars']) ? array_merge($_SESSION['__ci_vars'], $new) : $new;

			return true;
		}

		if (! isset($_SESSION[$key]))
		{
			return false;
		}

		$_SESSION['__ci_vars'][$key] = 'new';

		return true;
	}
public function markAsFlashdata($key): bool
	{
		if (is_array($key))
		{
			for ($i = 0, $c = count($key); $i < $c; $i ++)
			{
				if (! isset($_SESSION[$key[$i]]))
				{
					return false;
				}
			}

			$new = array_fill_keys($key, 'new');

			$_SESSION['__ci_vars'] = isset($_SESSION['__ci_vars']) ? array_merge($_SESSION['__ci_vars'], $new) : $new;

			return true;
		}

		if (! isset($_SESSION[$key]))
		{
			return false;
		}

		$_SESSION['__ci_vars'][$key] = 'new';

		return true;
	}
Mark a session property or properties as flashdata.

@param array|string $key Property identifier or array of them

@return boolean False if any of the properties are not already set