/**
* Subtracts part of the string.
*
* @param integer $start The starting point to extract from.
* @param integer $length The length to subtract from the string.
*
* @return String The substring (new instance of `String`).
*/
public function substr($start, $length = null)
{
return new static(static::$helper->substr($this->string, $start, $length));
}
