how to use the isset() command in code below for $_SESSION - php

Question

I have a basic question asking how to use the isset(); command and how it should be written in the example below. I am trying to insert values into query, then retrive the last ImageId inserted and insert that into another table
session_start();
$imagesql = "INSERT INTO Image (ImageFile)
VALUES (?)";
if (!$insert = $mysqli->prepare($imagesql)) {
// Handle errors with prepare operation here
}
//Dont pass data directly to bind_param store it in a variable
$insert->bind_param("s",$img);
//Assign the variable
$img = 'ImageFiles/'.$_FILES['fileImage']['name'];
$insert->execute();
if ($insert->errno) {
// Handle query error here
}
$insert->close();
$lastImageID = $mysqli->insert_id;
$_SESSION['lastImageID'] = $lastImageID;
$imagequestionsql = "INSERT INTO Image_Question (ImageId, SessionId, QuestionId)
VALUES (?, ?, ?)";
if (!$insertimagequestion = $mysqli->prepare($imagequestionsql)) {
// Handle errors with prepare operation here
echo "Prepare statement err imagequestion";
}
$qnum = (int)$_POST['numimage'];
$insertimagequestion->bind_param("iii",$lastImageID, $sessionid, $qnum);
$insertimagequestion->execute();
if ($insertimagequestion->errno) {
// Handle query error here
}
$insertimagequestion->close();

Answer 1

Not exactly sure what your asking for, but guessing wildly:
session_start();
...... //PDO code and commands
$lastImageID = $mysqli->insert_id;
$_SESSION['lastImageID'] = $lastImageID;
if (isset($_SESSION['lastImageID'])) {
// do this
} else {
// do that
}

Answer 2

You only need to call isset if you are reading a variable that may not be defined.
In this situation, you are writing to $_SESSION['lastImageID'], so you don't need to check.
An example of where isset would be needed:
if (isset($_SESSION['lastImageID']))
{
$lastImageID = $_SESSION['lastImageID'];
}
else
{
$lastImageID = get_this_from_db_or_something();
}