An if judge, looking for a day can not find an exception, only the

An if judgment, find a day can not find the mistake, only ask for help.
The code is as follows:

if ($webstate!== "Y" or $webstate!== "N") {
echo "";
Exit
}

My idea is: $ only two cases, one is Y, the other is n, if there is a third case, the error.

I now if only one condition, it is normal, if plus or more one condition, it is not normal (whether it is equal to Y or equal to n will error. ）
------to solve the idea----------------------
My idea is: $ only two cases, one is Y, the other is n, if there is a third case, the error.

According to you, the third case is not Y and not n, it should be with and not with OR.

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