Where do I define index?

Question

index.php contains links in this style:

<a href="index.php?id=1">1</a>
<a href="index.php?id=2">2</a>

and then a PHP passage in this style:

if ($_GET["id"] == "1") {}
elseif ($_GET["id"] == "2") {}

So when a link is clicked, the page is reloaded, and one of the if/elseif blocks is run.

However, when I load index.php the first time I get this error: Undefined index: id

I guess this means that $_GET["id"] needs to have a default value. But where do I put this default value in my code? If I give it, for example, the value "0" in the beginning of the script, the code blocks will never run, since the value keeps being reset on every reload.

Answer 1

You can first check to see if it has been set:

if (isset($_GET["id"])) {

    if ($_GET["id"] == "1") {

    } elseif ($_GET["id"] == "2") {

    }

}

isset is a language construct which will not issue any errors if the variable isn't set.

empty also won't issue errors but also checks for a "non-empty" value.