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 "add" or "delete" then have an error, else keep running the script. However, I get an error no matter what $action is.

 $action = $_GET['a'];
 if((!isset($action)) || ($action != "add" || $action != "delete")){
     //header("location:index.php");
     echo "error <br>";
 }

$action is being set properly and if run something like if($action =="add") it works. This is on my local host, so it could be a settings issue.

there is no sense in checking if $action is set, if you set it on the previous line — Your Common Sense
– Your Common Sense, Commented Jul 31, 2010 at 5:33
Shrapnel, if nothing is in the "?a=xxx" part of the url $action should be null. I think if I was setting $action to a static variable you would be right. Yet, because the user enters data there is a chance for problems. Correct me if I'm wrong. — Brooke.
– Brooke., Commented Jul 31, 2010 at 5:39
oops, my bad. isset() return false on null variables. Anyway you should check if $_GET['a'] set, not $action. Or you will get "Undefined index" error. — Your Common Sense
– Your Common Sense, Commented Jul 31, 2010 at 5:46
Can you explain the "index not set" error? I've never heard of that. — Brooke.
– Brooke., Commented Jul 31, 2010 at 5:48
error_reporting(E_ALL); <- just add this line to your config file/top of script and see what really happens in your code :) — Your Common Sense
– Your Common Sense, Commented Jul 31, 2010 at 5:53

John Kugelman · Accepted Answer · 2010-07-31 05:44:20Z

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".

Paige Ruten · Accepted Answer · 2010-07-31 05:31:57Z

12

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"))

answered Jul 31, 2010 at 5:31

Paige Ruten

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1 Comment

Brooke. Over a year ago

@dumbledor I think this was the easiest way to modify my original code and explain what I did wrong.

Artefacto · Accepted Answer · 2010-07-31 05:31:55Z

3

You're saying "if it's not set or it's different from add or it's different from delete". You realize that a != x && a != y, with x != y is necessarily false since a cannot be simultaneously two different values.

answered Jul 31, 2010 at 5:31

Artefacto

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Comments

Peter Mortensen · Accepted Answer · 2019-11-23 21:52:50Z

3

You could also try:

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

edited Nov 23, 2019 at 21:52

Peter Mortensen

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answered Jul 31, 2010 at 5:59

fastcodejava

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Comments

Peter Mortensen · Accepted Answer · 2019-11-23 21:51:21Z

1

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.

edited Nov 23, 2019 at 21:51

Peter Mortensen

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answered Jul 31, 2010 at 5:51

Your Common Sense

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Comments

Peter Mortensen · Accepted Answer · 2019-11-23 21:48:56Z

0

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

edited Nov 23, 2019 at 21:48

Peter Mortensen

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answered Jul 31, 2010 at 5:39

ina

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Comments

Peter Mortensen · Accepted Answer · 2019-11-23 21:50:01Z

0

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

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

edited Nov 23, 2019 at 21:50

Peter Mortensen

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answered Jul 31, 2010 at 5:42

Amit Sharma

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