Performs the same action as sUnion, but stores the result in the first key
执行一个并集操作就和sUnion()一样，但是结果储存在第一个参数中。
Parameters
Key: dstkey, the key to store the diff into.
存储结果的SET集合KEY
Keys: key1, key2, ... , keyN: Any number of keys corresponding to sets in redis.
求并集的KEYS
Return value
INTEGER: The cardinality of the resulting set, or FALSE in case of a missing key.
返回整数值：并集结果的个数。
Example
$redis->delete('s0', 's1', 's2');
$redis->sAdd('s0', '1');
$redis->sAdd('s0', '2');
$redis->sAdd('s1', '3');
$redis->sAdd('s1', '1');
$redis->sAdd('s2', '3');
$redis->sAdd('s2', '4');
var_dump($redis->sUnionStore('dst', 's0', 's1', 's2'));
var_dump($redis->sMembers('dst'));
Return value: the number of elements that are either in s0 or in s1 or in s2.
int(4)
array(4) {
[0]=>
string(1) "3"
[1]=>
string(1) "4"
[2]=>
string(1) "1"
[3]=>
string(1) "2"
}