Cannot Redirect Using $_SERVER Variables, PHP - php

Question

I have a PHP application (a request form) that first checks for an active $_SESSION before a user can access the site. Because of the timeout period set for this form there is rarely an active session. Here's the check:
if (isset($_SESSION['samlUserdata'])) {
$attributes = $_SESSION['samlUserdata'];
$user_department = $attributes['department'];
$user_email = $attributes['email'];
$user_employee_id = $attributes['employee_id'];
$user_full_name = $attributes['full_name'];
}
...and here is the else {} that I use to grab the REQUEST_URI:
else {
if (isset($_SERVER['REQUEST_URI'])) {
$referer = $_SERVER['REQUEST_URI'];
$redirect = "https://myinternalwebsite.net$referer";
}
header("Location: https://myinternalwebiste.net/confirm_auth.php?sso");
}
...and last, here is what I do with the $_GET
if (isset($_GET['sso'])) {
if (isset($redirect)) {
$auth->login($redirect);
} else {
$auth->login("https://myinternalwebsite.net/");
}
}
However, once my session is killed I am never properly routed back to the URL set in the ['REQUEST_URI'], I am always just dumped onto the internal site's front page. I have troubleshooted this on and off for some time over the last week, to no avail. I've tried other variables in the $_SERVER array as well, such as ['REDIRECT_URL'].
I'm at a loss, and I'm sure this fairly simple for anyone with more experience than myself... so I am all ears and eager to learn.
EDIT:
Thank you for the comments below. Per your advice I will add the entirety of my code here, removing only the unnecessary parts. (And yes, I appreciate the tip to flip the initial (isset()) to (!isset(). Thank you for that.)
<?php
session_start();
$auth = new OneLogin\Saml2\Auth($saml_settings);
if (isset($_SESSION['samlUserdata'])) {
$attributes = $_SESSION['samlUserdata'];
$user_department = $attributes['department'];
$user_email = $attributes['email'];
$user_employee_id = $attributes['employee_id'];
$user_full_name = $attributes['full_name'];
} else {
if (isset($_SERVER['REQUEST_URI'])) {
$referer = $_SERVER['REQUEST_URI'];
$redirect = "https://example.net$referer";
}
header("Location: https://example.net/confirm_auth.php?sso");
}
if (isset($_GET['sso'])) {
if (isset($redirect)) {
$auth->login($redirect);
} else {
$auth->login("https://example.net/");
}
} else if (isset($_GET['slo'])) {
$auth->logout();
} else if (isset($_GET['acs'])) {
$auth->processResponse();
$errors = $auth->getErrors();
if (!empty($errors)) {
echo '<p>', implode(', ', $errors), '</p>';
}
if (!$auth->isAuthenticated()) {
echo "<p>Not authenticated!</p>";
exit();
}
$_SESSION['samlUserdata'] = $auth->getAttributes();
if (isset($_POST['RelayState']) &&
OneLogin\Saml2\Utils::getSelfURL() != $_POST['RelayState']) {
$auth->redirectTo($_POST['RelayState']);
}
} else if (isset($_GET['sls'])) {
$auth->processSLO();
$errors = $auth->getErrors();
if (empty($errors)) {
echo '<p>Sucessfully logged out!</p>';
} else {
echo '<p>', implode(', ', $errors), '</p>';
}
}
?>