Returns the rank of a given member in the specified sorted set, starting at 0 for the item with the smallest score. zRevRank starts at 0 for the item with the largest score.
返回key对应的有序集合中member元素的索引值，元素按照score从低到高进行排列。rank值（或index）是从0开始的，这意味着具有最低score值的元素的rank值为0。使用ZREVRANK可以获得从高到低排列的元素的rank（或index）。
Parameters
key
member
Return value
Long, the item's score.
Example
$redis->delete('z');
$redis->zAdd('key', 1, 'one');
$redis->zAdd('key', 2, 'two');
$redis->zRank('key', 'one'); /* 0 */
$redis->zRank('key', 'two'); /* 1 */
$redis->zRevRank('key', 'one'); /* 1 */
$redis->zRevRank('key', 'two'); /* 0 */
