PHP if not statements

Question

This may be the way my server is set up, but I\'m banging my head against the wall. I\'m trying to say that if $action has no value or has a value that is not \

Answer 1

Your logic is slightly off. The second || should be &&:

if ((!isset($action)) || ($action != "add" && $action != "delete"))

You can see why your original line fails by trying out a sample value. Let's say $action is "delete". Here's how the condition reduces down step by step:

// $action == "delete"
if ((!isset($action)) || ($action != "add" || $action != "delete"))
if ((!true) || ($action != "add" || $action != "delete"))
if (false || ($action != "add" || $action != "delete"))
if ($action != "add" || $action != "delete")
if (true || $action != "delete")
if (true || false)
if (true)

Oops! The condition just succeeded and printed "error", but it was supposed to fail. In fact, if you think about it, no matter what the value of $action is, one of the two != tests will return true. Switch the || to && and then the second to last line becomes if (true && false), which properly reduces to if (false).

There is a way to use || and have the test work, by the way. You have to negate everything else using De Morgan's law, i.e.:

if ((!isset($action)) || !($action == "add" || $action == "delete"))

You can read that in English as "if action is not (either add or remove), then".

Answer 2

You could also try:

if ((!isset($action)) || !($action == "add" || $action == "delete")) {
  // Do your stuff
}

Answer 3

For future reference, you can quickly create a truth table to check if it evaluates the way you want... it's kind of like Sudoku.

(!isset($action)) && ($action != "add" && $action != "delete"))

Example:

column 1 is issetaction, column 2 and 3 evaluates !="add","delete" respectively

if($a=add) T && (F && T) => T && F => FALSE

if($a=delete) T && (T && F) => T && F => FALSE

if($a=nothing) T && (T && T) => T && T => TRUE

Answer 4

No matter what $action is, it will always either not be "add" OR not be "delete", which is why the if condition always passes. What you want is to use && instead of ||:

(!isset($action)) || ($action !="add" && $action !="delete"))

Answer 5

Not an answer, but just for the sake of code formatting

if((isset($_GET['a'])) $action=$_GET['a']; else $action ="";
if(!($action === "add" OR $action === "delete")){
  header("location: /index.php");
  exit;
}

Note the exit; statement after header(). That's the important thing. header() does not terminate script execution.

Answer 6

I think this is the best and easiest way to do it:

if (!(isset($action) && ($action == "add" || $action == "delete")))