Sets the user variable $x with str. Note that str can contain other variables, or the user variable being set (concatenating strings this way is possible, etc). If maxlen is specified, the string will be a maximum of maxlen characters (if maxlen is negative, the string will be truncated abs(maxlen) characters from the end). If start_offset is specified, the source is offset by it (if start_offset is negative, it will start abs(start_offset) from the end of the string).
