So this may be the way my server is set up but I'm banging my head agianst the wall. What I'm trying to do is say that if $action has no value or has a value that is not "add" or "delete" than 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") ){
   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 could be a setting issue.

+6  A: 

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".

John Kugelman

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.

+5  A: 

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" && $action !="delete"))
sorry , i donot think this is right way
Extjs Commander
@dumbledor I think this was the easiest way to modify my original code and explain what I did wrong.
if( !( isset($action)  && ($action =="add" || $action =="delete" )) )

i think this is the best and easiest way to do it

Extjs Commander

not an answer but just in sake of code formatting

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

note exit; statement after header(). that's important thing. header() do not terminate script execution

Col. Shrapnel
+1  A: 

You could also try :

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