PHP $_SESSION not working in function - php

Question

Firstly, I have searched many threads and topics and they all keep saying its function placement but thus far I do not see a issue with my placement. I am desperate to get this is working because I am SICK of looking at 50+ extra lines of repetitive code.
resetpassword.php (RELEVANT CODE):
<?php
require_once $_SERVER['DOCUMENT_ROOT'] . '/php/security/sslcheck.php';
$ResetID = $_GET["ID"];
session_start();
require_once $_SERVER['DOCUMENT_ROOT'] . '/php/functionality/error.php';
print_r(array_values($_SESSION));
if(empty($ResetID) && !isset($_SESSION["ERR"]) && !isset($_SESSION["ERR_MSG"])) {
$ResetPassword = "REQUEST";
if(isset($_POST["email-search"]) && !empty($_POST["email-search"])) {
require_once $_SERVER['DOCUMENT_ROOT'] . '/php/functionality/users/request.php';
// Start Request Process
$Request = RequestPasswordReset($_POST["email-search"]);
if($Request === "USER_NOT_FOUND") {
DisplayError("Password Reset Request Failed", "The specified email is not tied to any accounts in our system.", "/account/resetpassword.php");
} else if ($Request === "MAIL_FAILURE") {
DisplayError("Password Reset Request Failed", "Failed to email you the password reset email.", "/account/resetpassword.php");
} else {
DisplayError("Password Reset Request Success", "We have sent a email that contains a link to reset your password.", "/account/resetpassword.php");
}
}
More Relevant Code
<?php
if (isset($_SESSION["ERR"]) && isset($_SESSION["ERR_MSG"])) {
echo '<div id="resetPasswordStatus">';
echo '<h4>' . $_SESSION["ERR"] . '</h4>';
echo '<p>' . $_SESSION["ERR_MSG"] . '</p>';
echo '</div>';
session_destroy();
}
?>
error.php (ALL CODE):
<?php
session_start();
function DisplayError($title, $body, $returnlink) {
$_SESSION["ERR"] = $title;
$_SESSION["ERR_MSG"] = $body;
header("Location: " . $returnlink);
}
?>
I have experiment in many ways with my placement of require_once of error.php, but have found no luck. I understand $_SESSION is a superglobal and require or require_once copy the code right into place where required. However even copying error.php manually into resetpassword.php I was unable to get the function to work. Thank you guys for your help as it really means a lot!
The expected output is after the callback after the request for a password reset, is the alert to display. Code for this alert can be seen under More Relevant Code.

Answer 1

Solution:
In the error.php if you add exit(); after the header redirect it allows for PHP to pause execution while the redirect occurs which prevents the error displaying code from running its code and then wiping the session which leaves the redirect empty in terms of sessions.

Answer 2

You are calling the function after the if statement empty($ResetID)
Your function isn't running if the $_GET['ID'] is not empty so change it to !empty($ResetID)