Undefined index PHP placing at host - php

Question

I have to make a site for school. It need to be linked with a database. On localhost everything is ok and works, but when I tried to upload it on the host of the school, I get this error:
Notice: Undefined index: vragen in /mnt/studentenhomes/arnaud.gandibleux/public_html/datamanagement/index.php on line 44
I can't find the solution
index.php
<div id="tekst">
<table align='center'>
<?php
//if (isset($_GET['vragen'])){
if ($_GET['vragen'] === 'Alleclubs') {
getclubs();
} elseif ($_GET['vragen'] === 'ledenvjf') {
getVJFleden();
echo "test";
} elseif ($_GET['vragen'] === 'ledenffbj') {
getFFBJleden();
}
elseif (isset($_GET['clubnr'])) {
getLedenPerClubEnID($_GET['clubnr']);
}
else{
getclubs();
}
// }
?>
</table>
Crud.php
function getVJFleden() {
global $mysqli;
$result = $mysqli->query("SELECT * FROM Leden l JOIN Clubs c ON l.clubnr = c.clubnr join Bonden b
ON b.ID_bond = c.ID_bond LEFT JOIN adressen a ON a.ID_adress = l.ID_adress WHERE b.naam_bond = 'VJF';");
if ($result) {
if ($result->num_rows > 0) {
echo"<caption>Alle leden VJF</caption>";
echo "<th>Voornaam</th><th>achternaam</th><th>leeftijd</th><th>Kye</th><th>adress</th>";
while ($leden = $result->fetch_object()) {
echo "<tr><td>$leden->lid_voornaam</td> ";
echo "<td>" . $leden->lid_achternaam . "</td> ";
echo "<td>" . $leden->lid_leeftijd . "</td> ";
echo "<td>" . $leden->kye . "</td> ";
echo "<td>" . $leden->straatnaam . " " . $leden->huisnummer . " " . $leden->postcode . " " . $leden->gemeente . "</td> ";
echo "<td><form id='update' action='update.php' method='POST'>
<input type='hidden' name='id' value='" . $leden->ID_lid . "'/>
<input type='hidden' name='clubnr' value='" . $_GET['clubnr'] . "'/>
<input type='image' src='update.png' alt='Update' width='22' height='22'>
</form>
<form id='delete' action='deleteLid.php' method='POST'>
<input type='hidden' name='id' value='" . $leden->ID_lid . "'/>
<input type='image' src='delete.png' alt='detele' width='22' height='22'>
</form></td> ";
}
}
}
$mysqli->close();
}
function getFFBJleden() {
global $mysqli;
#, Adressen a
#AND l.ID_adress = a.ID_adress
$result = $mysqli->query("SELECT * FROM Leden l JOIN Clubs c ON l.clubnr = c.clubnr join Bonden b ON b.ID_bond = c.ID_bond LEFT JOIN adressen a ON a.ID_adress = l.ID_adress WHERE b.naam_bond = 'FFBJ';");
if ($result) {
if ($result->num_rows > 0) {
echo"<caption>Alle leden VJF</caption>";
echo "<th>Voornaam</th><th>achternaam</th><th>leeftijd</th><th>Kye</th><th>adress</th>";
while ($leden = $result->fetch_object()) {
echo "<tr><td>$leden->lid_voornaam</td> ";
echo "<td>" . $leden->lid_achternaam . "</td> ";
echo "<td>" . $leden->lid_leeftijd . "</td> ";
echo "<td>" . $leden->kye . "</td> ";
echo "<td>" . $leden->straatnaam . " " . $leden->huisnummer . " " . $leden->postcode . " " . $leden->gemeente . "</td> ";
echo "<td><form id='update' action='update.php' method='POST'>
<input type='hidden' name='id' value='" . $leden->ID_lid . "'/>
<input type='hidden' name='clubnr' value='" . $_GET['clubnr'] . "'/>
<input type='image' src='update.png' alt='Update' width='22' height='22'>
</form>
<form id='delete' action='deleteLid.php' method='POST'>
<input type='hidden' name='id' value='" . $leden->ID_lid . "'/>
<input type='image' src='delete.png' alt='detele' width='22' height='22'>
</form></td> ";
}
}
}
$mysqli->close();
}

Answer 1

You need to ensure that the array index exists before you try to use it. Being that it is a $_GET variable, it may not have been passed as a URL parameter.
Uncomment
//if (isset($_GET['vragen'])){
To
if (isset($_GET['vragen'])){

Answer 2

Two things are happening:
you have Notice warnings on. See here on how to turn them off: (How do I turn off PHP Notices?)
It's probably one of the $_GET parameters that you didn't specify on the url. So when you load the page, you're accessing the $_GET superglobal, but there may be nothing in it, and you're trying to print out that value.
You can either turn off notices or use a function to get a values from $_GET, and in this function check that the value isset() first before you access it.

Answer 3

when you visit the site you do something like: mysite.com?vragen=ledenvjf
if you forget the part after the ? there is no $_GET['vragen'] and the notice is thrown
this line
//if (isset($_GET['vragen'])){
prevents the notice from being thrown you should uncomment it together with this line:
// }

Answer 4

It means that variable vragen is not set so your $_GET array doesn't have any element with index vragen.
Uncomment //if (isset($_GET['vragen'])){ and // } since this was checking if $_GET variable with name vragen was set. Example of when this check will pass index.php?vragen=ledenvjf