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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.
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A simple negation is to add parenthesis and negate the complete expression. Negation of a logical expression means, to negate all elements, and exchange && and || . For priority reasons, you have to add parenthesis (like here) or you can remove them.
Introduction to PHP not equal. One of the comparison operators in PHP is not equal, which is represented by the symbol != or <> and whenever we want to compare the data types of the two given values, we make use of not equal operator in PHP.
PHP Conditional Statements if statement - executes some code if one condition is true. if...else statement - executes some code if a condition is true and another code if that condition is false. if... elseif...else statement - executes different codes for more than two conditions.
An if statement by its self is absolutely fine. If you have a series of conditions where only one will be true it's better to put the most likely first, then use else if on the subsequent ones so that PHP doesn't have to keep evaluating each condition.
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".
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