Not operator with multiple or operator - php

Question

I am confuse with bellow code:
Fist code that is not working:
if ($_SESSION['userType'] != 'A' || $_SESSION['userType'] != 'S')
{
redirect('dashboard.php');
}
Second code that is working:
if (!($_SESSION['userType'] == 'A' || $_SESSION['userType'] == 'S'))
{
redirect('dashboard.php');
}
What is difference between both code?

Answer 1

I know you already have six answers to your question, but none of them uses plain English, so I'll make an attempt myself as well.
Let's look at this code step by step, splitting it up into smaller portions, and explaining each step exactly.
Code that isn't working
First you have this row:
if ($_SESSION['userType'] != 'A' || $_SESSION['userType'] != 'S')
Instead of looking at the full if clause already, let's divide it into two parts:
if ($_SESSION['userType'] != 'A')
if ($_SESSION['userType'] != 'S')
The first part will trigger any time that $_SESSION['userType'] is any value other than A.
The second part will trigger any time that $_SESSION['userType'] is any value other than S.
Now, you've joined them both together with an "or" operator, so that it's enough that one of them is true for the if clause to trigger. Let's see what happens when we try it out.
We set $_SESSION['userType'] to 'B' and go into the if clause. The first thing that happens is that PHP looks at the first part, if ($_SESSION['userType'] != 'A'), and finds that 'B' != 'A'. It doesn't need to go to the second part, because it already found that one of the parts of the if statement is true, and so it decides that the full if statement must also be true.
We set $_SESSION['userType'] to 'A' and go into the if clause. The first thing that happens is that PHP looks at the first part, if ($_SESSION['userType'] != 'A'), and finds that 'A' == 'A', so the first part is false. It then goes on to the second part, which is if ($_SESSION['userType'] != 'S'), and finds that 'A' != 'S'. Since this means that one of the parts of the if statement is true, it again decides that the full if statement must also be true.
Now notice what this means. Even if you send an 'A' to the if clause, it will still trigger because of the second part. And if you were to send an 'S', it will trigger because of the first part. So essentially, the full first if statement will trigger regardless of what $_SESSION['userType'] is set to. It's a tautology - it's always true.
Code that is working
This is your full if statement:
if (!($_SESSION['userType'] == 'A' || $_SESSION['userType'] == 'S'))
Let's ignore the ! for now, and divide it into the two parts of the or operator.
if ($_SESSION['userType'] == 'A')
if ($_SESSION['userType'] == 'S')
Notice that this is the logical opposite of the code you had that didn't work. The first row means "only accept an 'A'" and the second means "only accept an 'S'". So this code will trigger only if you give it either an 'A', or an 'S'.
Now, what happens if you put an ! in front of the whole thing, like you did?
The ! operator simply reverses whatever it is put in front of. So this changes the meaning from "either A or S" to "neither A nor S".
Summary
Your first example comes down to "any value whatsoever".
Your second example comes down to "any value, but not A, and also not S".
If you want to learn more of basic logic, I suggest looking at De Morgan's laws, as you were linked to in an above comment. This will give you an understanding of how and and or fits together.

Answer 2

Not...quite.
If I remember boolean logic correctly, the fault lies in order of operations.
if ($_SESSION['userType'] != 'A' || $_SESSION['userType'] != 'S') {
if userType equals A, this will not work...since the first operation is to check that it is not equal to A and ORs are left to right...so if the first is not true, the entire statement is false.
Your second approach translates to:
if (!($_SESSION['userType'] == 'A' || $_SESSION['userType'] == 'S'))
{
redirect('dashboard.php');
}
IF NOT userType equals A OR userType equals S which is why it works as you desire it to.
Try
if ($_SESSION['userType'] != 'A' && $_SESSION['userType'] != 'S')) {
which means IF userType is not A and userType is not S
I think that will give you what you are probably looking for. IE Only do what is in the loop if the userType is neither A, nor S.

Answer 3

if ($_SESSION['userType'] != 'A' || $_SESSION['userType'] != 'S') {
is more or less equivalent to
if (!($_SESSION['userType'] == 'A' && $_SESSION['userType'] == 'S')) {

Answer 4

first code that is not working
if ($_SESSION['userType'] != 'A' || $_SESSION['userType'] != 'S')
{
redirect('dashboard.php');
}
lets simplyfy it as under:
if (true || true) //the condition will be true or vice versa
{
redirect('dashboard.php');
}
Second code that is working:
if (!($_SESSION['userType'] == 'A' || $_SESSION['userType'] == 'S'))
{
redirect('dashboard.php');
}
and the simplified version:
if (!(true || true)) same as if (!(true)) //i.e. false or vice versa
{
redirect('dashboard.php');
}
so basically one is true and the other is false or vice versa so thats why they are acting diffrenetly.