It normally takes two parameters: start and length. It can also take one parameter: start, i.e. length is omitted, in which case the substring starting from start until the end of the string will be returned.
To extract characters from the end of the string, use a negative start number.
In addition, borrowing from the extended behavior described at http://php.net substr, if length is given and is negative, that number of characters will be omitted from the end of string.