Description
Sets an expiration date (a timestamp) on an item. pexpireAt requires a timestamp in milliseconds.
给一个KEY设置一个生命周期，单位是UNIX的时间戳。（pexpireAt与其类似，区别是单位为毫秒）
Parameters
Key: key. The key that will disappear.
Key:将被销毁的key
Integer: Unix timestamp. The key's date of death, in seconds from Epoch time.
integer:Unix 时间戳
Return value
BOOL: TRUE in case of success, FALSE in case of failure.
Example
$redis->set('x', '42');
$now = time(NULL); // current timestamp
$redis->expireAt('x', $now + 3); // x will disappear in 3 seconds.
sleep(5); // wait 5 seconds
$redis->get('x'); // will return `FALSE`, as 'x' has expired.
