Returns a random element from the set value at Key, without removing it.
取得指定SET容器中的一个随机元素，但不会在SET容器中移除它。
Parameters
key
Return value
String value from the set
Bool FALSE if set identified by key is empty or doesn't exist.
Example
$redis->sAdd('key1' , 'member1');
$redis->sAdd('key1' , 'member2');
$redis->sAdd('key1' , 'member3'); /* 'key1' => {'member3', 'member1', 'member2'}*/
$redis->sRandMember('key1'); /* 'member1', 'key1' => {'member3', 'member1', 'member2'} */
$redis->sRandMember('key1'); /* 'member3', 'key1' => {'member3', 'member1', 'member2'} */