undefined offset PHP, not sure what it does [duplicate] - php

Question

This question already has answers here:
"Notice: Undefined variable", "Notice: Undefined index", "Warning: Undefined array key", and "Notice: Undefined offset" using PHP
(29 answers)
Closed 8 months ago.
I have the following error in PHP error.log
[Tue Dec 19 12:08:22.887574 2017] [:error] [pid 32196] [client xx.xx.xx.x:20560] PHP Notice: Undefined offset: 8 in /var/www/html/page.php on line 55, referer: view.php?1x=8
and the php code that i think causes this is:
$i = isset($_SESSION['i']) ? $_SESSION['i'] : 0;
// echo $_SESSION['websites'][$i];
$website = explode(";", $_SESSION['websites'][$i]);
// echo $website[0];
$i++;
$_SESSION['i'] = $i;
I dont really know what $i = isset($_SESSION['i']) ? $_SESSION['i'] : 0; does
thank you
session_start();
$i = isset($_SESSION['i']) ? $_SESSION['i'] : 0;
if ($_SESSION['i'] < $sesioni1x) {
// echo $_SESSION['websites'][$i];
$website = explode(";", $_SESSION['websites'][$i]);
// echo $website[0];
$i++;
$_SESSION['i'] = $i;
header("Location: $website[0]"); //redirect
die();
// echo $website[0];
// echo $sesioni1x;
// echo $website[0]." Frame-URL<br>";
// $_SESSION['actual_website'] = $website[0];
}
if ($_SESSION['i'] == $sesioni1x) {
$handle = fopen($list1x, "a"); //open file to append content to csv file
fputcsv($handle, $_SESSION['addwebsite'], ";"); //insert line in opened file
fclose($handle); //close file
header("Location: index.php"); //redirect
die();
// echo "session = var";
}
this is the full code where i get this warning, i must say that script is doing his job, but i want to get rid of the error
the $_SESSION['i'] in the source is 0

Answer 1

this is ternary called ternary operator. Basically it is a simple if/else.
$i = (isset($_SESSION['i']) ? $_SESSION['i'] : 0);
To help you understand it's this code in 1 line
if(isset($_SESSION['i'])){
$i=$_SESSION['i'];
}
else{
$i=0;
}

Answer 2

$i = isset($_SESSION['i']) ? $_SESSION['i'] : 0;
The above code is just an if/else nothing else
this will check for the $_SESSION['i'] is it is set then assign it's value to $i otherwise it assigns 0 to $i variable.
Now looking to your problem, it looks like your $_SESSION['i'] is already set and its value is 8 so $i = 8,
Now, $website = explode(";", $_SESSION['websites'][$i]); this like is checking for your $_SESSION['website'] array and trying to find 8th element from the array which has not been set so it gives an error its undefined.

Answer 3

(e) ? r1 : r2 is a conditional statement.
If the expression e is true the value is r1, if e is false the value is r2. So
$i = isset($_SESSION['i']) ? $_SESSION['i'] : 0;
means "i is $_SESSION['i'], if $_SESSION['i'] is set, otherwise it is 0".