PHP login problem

1、 Set up cookie

2、session

3、 Distinguish

4、isset
The official explanation is ： Checks whether the variable is set and not NULL.
It should be noted that , What's not here NULL Not null .
So let's see isset() The return value of ：
1. Returns... If the variable does not exist FALSE
2. If the variable exists and its value is NULL, Also returned FALSE
3. If the variable exists and the value is not NULL, Then return to TURE
5、cookie encryption