获取字符串的子串. 若 start 是负数, 则从字符串末尾算起. 若 size 是负数, 则表示从字符串末尾算起, 忽略掉那么多字节, 类似 PHP 的 substr().
key - start - Optional, the offset of first byte returned. If start is negative, the returned string will start at the start'th character from the end of string.size - Optional, number of bytes returned. If size is negative, then that many characters will be omitted from the end of string.The extracted part of the string.