Performs the same action as sDiff, but stores the result in the first key
与sDiff函数功能一直，只是结果为一个新的SET集合，存储到dstkey。
Parameters
Key: dstkey, the key to store the diff into.
Key:存储结果的SET集合KEY
Keys: key1, key2, ... , keyN: Any number of keys corresponding to sets in redis
参与操作的SET集合
Return value
INTEGER: The cardinality of the resulting set, or FALSE in case of a missing key.
返回整数：为结果集的个数。
Example
$redis->delete('s0', 's1', 's2');
$redis->sAdd('s0', '1');
$redis->sAdd('s0', '2');
$redis->sAdd('s0', '3');
$redis->sAdd('s0', '4');
$redis->sAdd('s1', '1');
$redis->sAdd('s2', '3');
var_dump($redis->sDiffStore('dst', 's0', 's1', 's2'));
var_dump($redis->sMembers('dst'));
Return value: the number of elements of s0 that are neither in s1 nor in s2.
int(2)
array(2) {
[0]=>
string(1) "4"
[1]=>
string(1) "2"
}