Description
Watches a key for modifications by another client. If the key is modified between WATCH and EXEC, the MULTI/EXEC transaction will fail (return FALSE).
监控一个KEY是否被其他的客户端修改。如果KEY在调用watch()和exec()之间被修改，那么批量处理最终的exec()执行将失败。通过一些实验，这个函数的效果其实并没有那么好，或者说不能够准确的去监控。
Parameters
keys: a list of keys
Example
$redis->watch('x');
/* long code here during the execution of which other clients could well modify `x` */
$ret = $redis->multi()
->incr('x')
->exec();
/*
$ret = FALSE if x has been modified between the call to WATCH and the call to EXEC.
*/