system/Session/Session.php
public function setFlashdata($data, $value = null)
	{
		$this->set($data, $value);
		$this->markAsFlashdata(is_array($data) ? array_keys($data) : $data);
	}
public function setFlashdata($data, $value = null)
	{
		$this->set($data, $value);
		$this->markAsFlashdata(is_array($data) ? array_keys($data) : $data);
	}
Sets data into the session that will only last for a single request.
Perfect for use with single-use status update messages.

If $data is an array, it is interpreted as an associative array of
key/value pairs for flashdata properties.
Otherwise, it is interpreted as the identifier of a specific
flashdata property, with $value containing the property value.

@param array|string $data  Property identifier or associative array of properties
@param string|array $value Property value if $data is a scalar