给KEY设置一个生存周期，单位为秒。（pexpire类似，它是使用毫秒作为计算单位）
Parameters
Key: key. The key that will disappear.
Key:将被销毁的KEY
Integer: ttl. The key's remaining Time To Live, in seconds.
integer:生命周期
Return value
BOOL: TRUE in case of success, FALSE in case of failure.
Example
$redis->set('x', '42');
$redis->expire('x', 3); // x will disappear in 3 seconds.
sleep(5); // wait 5 seconds
$redis->get('x'); // will return `FALSE`, as 'x' has expired.
