对指定的KEY的值自减1(如果要自减指定的值，可以使用decrBy)
Parameters
key
value: value that will be substracted to key (only for decrBy)
Return value
INT the new value
Examples
$redis->decr('key1'); /* key1 didn't exists, set to 0 before the increment */
/* and now has the value -1 */
$redis->decr('key1'); /* -2 */
$redis->decr('key1'); /* -3 */
