PHP 为什么我的“if”语句总是给出 TRUE？

Question

I have been making stuff in PHP for a while, everything is always fine. But today I don't get this statement. Why is it always true?

if ($action != 1 || $action != 2) echo true; // TRUE for 0, 1, 2, 3

But the reverse logic

if ($action == 1 || $action == 2) echo true; // FALSE for 0, 3 TRUE for 1, 2

The first expression blows my mind. I guess I don't understand something very very basic, not in PHP but in the Universe, so I don't get it here. I thought that if (FALSE || TRUE) == FALSE , but it isn't a case for second example. It works as expected.

So, where is the answer how to say that: "If the variable is not 1 OR 2 - echo true". I don't understand why my if ($var != 1 OR $var != 2) echo true; doesn't work as I expect.

Answer 1

Negation of ($action != 1 || $action != 2) is ($action == 1 && $action == 2) . You can see for yourself that the latter is always false because variable can not be both 1 and 2 at the same time. Therefore the original condition is bound to be always true.

Answer 2

It is working as it has to work. See the doc http://php.net/manual/en/language.operators.logical.php

$a || $b $a || $b returns TRUE if either $a or $b is TRUE .

If you try like this, hope it will make sense, see the comment on every line

$action = 0;
var_dump($action != 1 || $action != 2); //here (true || true)
$action = 1;
var_dump($action != 1 || $action != 2); //here (false || true)
$action = 2;
var_dump($action != 1 || $action != 2); //here (true || false)
$action = 3;
var_dump($action != 1 || $action != 2); //here (true || true)

Answer 3

Condition OR will search for first TRUE result, your code will give TRUE all the time as any value will be even not 1 or 2. From you example if $action = 1 then the condition $action != 2 will give true, also if $action = 2 then $action != 1 will give TRUE.

For (If the variable is not 1 OR 2 - echo true) use this:

if(!in_array($action, array(1,2)) echo "true";

EDIT: You can also check it like this:

if(!($action == 1 || $action == 2)) echo "true";

Answer 4

    if ($action == 2 || $action == 1) echo true; // FALSE for 0, 3 TRUE for 1, 2

Works as intended. Its basically "echo true if $action is either 1 or 2".