Performs the union between N sets and returns it.
执行一个并集操作在N个SET容器之间，并返回结果。
Parameters
Keys: key1, key2, ... , keyN: Any number of keys corresponding to sets in redis.
Return value
Array of strings: The union of all these sets.
返回一个数组
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->sUnion('s0', 's1', 's2'));
Return value: all elements that are either in s0 or in s1 or in s2.
array(4) {
[0]=>
string(1) "3"
[1]=>
string(1) "4"
[2]=>
string(1) "1"
[3]=>
string(1) "2"
}