Notice: Undefined Index in Xampp. How do I fix this?

Question

Before you ask, yes I have searched through nearly 20 different "undefined index" questions on SO before asking this question. Sadly none of them managed to even give me a clue to fixing the problem. I have a simple index.php file that switches through different html files depending on what link you press. On a live server that seems to be no problem at all but when I use Xampp, I receive a long list of "undefined indexes" and I can't figure out why. My Index.php file below. The undefined index message states that "Undefined Index rv in each line."

<?php
include ("anime_header.html");

if($_GET['rv'] == "amnesia")         include ("Amnesia.html");
else if($_GET['rv'] == "shingeki")   include ("AttackOnTitan.html");
else if($_GET['rv'] == "chuunibyou") include ("Chuunibyou.html");
else if($_GET['rv'] == "crimeedge")  include ("CrimeEdge.html");
else if($_GET['rv'] == "datealive")  include ("DateALive.html");
else if($_GET['rv'] == "duskmaiden") include ("DuskMaiden.html");
else if($_GET['rv'] == "gargantia")  include ("Gargantia.html");
else if($_GET['rv'] == "K_anime")    include ("K_Anime.html");
else if($_GET['rv'] == "karneval")   include ("Karneval.html");
else if($_GET['rv'] == "kotoura")    include ("Kotoura-San.html");
else if($_GET['rv'] == "kaibutsu")   include ("LittleMonster.html");
else if($_GET['rv'] == "nerawareta") include ("Nerewareta.html");
else if($_GET['rv'] == "redgarden")  include ("RedGarden.html");
else if($_GET['rv'] == "saikano")    include ("Saikano.html");
else if($_GET['rv'] == "sakurasou")  include ("Sakurasou.html");
else if($_GET['rv'] == "sasamisan")  include ("Sasami-San.html");
else if($_GET['rv'] == "vividred")   include ("Vividred.html");
else include ("animereviews.html");

include ("ReviewFooter.html");
?>

I'm not looking for a way to suppress the error message but an understanding as to why it appears in on my Xampp server only and how to fix it in the future.

Answer 1

Check if $_GET['rv'] is set

<?php
$rv = $_GET['rv'];
if(isset($rv)){
    include ("anime_header.html");

    if($rv == "amnesia")         include ("Amnesia.html");
    else if($rv == "shingeki")   include ("AttackOnTitan.html");
    else if($rv == "chuunibyou") include ("Chuunibyou.html");
    else if($rv == "crimeedge")  include ("CrimeEdge.html");
    else if($rv == "datealive")  include ("DateALive.html");
    else if($rv == "duskmaiden") include ("DuskMaiden.html");
    else if($rv == "gargantia")  include ("Gargantia.html");
    else if($rv == "K_anime")    include ("K_Anime.html");
    else if($rv == "karneval")   include ("Karneval.html");
    else if($rv == "kotoura")    include ("Kotoura-San.html");
    else if($rv == "kaibutsu")   include ("LittleMonster.html");
    else if($rv == "nerawareta") include ("Nerewareta.html");
    else if($rv == "redgarden")  include ("RedGarden.html");
    else if($rv == "saikano")    include ("Saikano.html");
    else if($rv == "sakurasou")  include ("Sakurasou.html");
    else if($rv == "sasamisan")  include ("Sasami-San.html");
    else if($rv == "vividred")   include ("Vividred.html");
    else include ("animereviews.html");

    include ("ReviewFooter.html");
}
?>

Answer 2

That error appears when you try to access a non-existing key in an array. As an example, let's make a simple array:

$myarr = array(
    'a' => 5
);

If you try to access the "a" key, it will run without any errors:

if ($myarr['a'] == 5){
    echo 'Perfect!';
}

However, if you try to access a key that has not been set, a "Undefined Index" error will occur:

if ($myarr['b'] == 6){
    echo 'The previous line should cause an error.';
}

In order to fix this, ensure that the "rv" key exists before accessing it. The most commonly used function to ensure that a key is set is isset:

if(!isset($_GET['rv'])){
    echo 'Oops, it seems like that key is not set!';
    exit;
}