Removes and returns a random element from the set value at Key.
随机返回一个元素，并且在SET容器中移除该元素。
Parameters
key
Return value
String "popped" value
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->sPop('key1'); /* 'member1', 'key1' => {'member3', 'member2'} */
$redis->sPop('key1'); /* 'member3', 'key1' => {'member2'} */
