Session variable being destroyed after migrating to live website - php

Question

I've just migrated my website from my local WAMP server to a live HTTPS server on 1and1. It works perfectly on the local server, but on the live server the session variables are being destroyed when I'm trying to log in. I know the database is working fine and all the queries are running successfully after some testing.
The problem is that the session variable is being created when I run the log in script, but once the page reloads and runs a 'session check', the variable no longer exists. Because of this the site just reloads the login form as the if condition is not being met.
Here is the code for both scripts. I don't know why this is happening as the entire website is being run through HTTPS so its not an issue with HTTP/HTTPS etc.
LOGIN SCRIPT
<?php
date_default_timezone_set("Europe/London");
require("db_connect.php");
if ($sql)
{
$email = $_POST['userEmail'];
$password = $_POST['userPassword'];
$checkDetails = mysqli_query($sql, "SELECT * FROM users WHERE email='$email'");
while ($details = mysqli_fetch_array($checkDetails))
{
$hashedPassword = $details['password'];
if(password_verify($password, $hashedPassword))
{
//Passwords Match
//Update last login time in the database
$now = date("Y-m-d H:i:s");
$lastLoginQuery = mysqli_query($sql, "UPDATE users SET lastLogin='$now' WHERE email='$email'");
if ($lastLoginQuery)
{
//Initialise session
session_start();
$_SESSION['user'] = $email;
header("Location: ../");
}
else
{
echo "There was an error logging you in. Please try again!";
}
}
else
{
echo "The details you entered are incorrect. Please return to the login page. If the problem persists, contact an administrator.";
}
}
}
else
{
echo "There was a problem connecting to the database";
}
?>
SESSION CHECKING SCRIPT
<?php
//Check if a session exists and load the page if it does
session_start();
if (isset($_SESSION['user']))
{
//Check if the session has timed out
if (isset($_SESSION['LAST_ACTIVITY']) && (time() - $_SESSION['LAST_ACTIVITY'] > 1800))
{
//Last user action performed more than 30 minutes ago. Log out now.
session_unset();
session_destroy();
header("Location:./");
}
//If the session hasnt timed out. Reset the last activity time.
$_SESSION['LAST_ACTIVITY'] = time();
//Continue to load content
include('./includes/main.php');
}
else
{
// Load login page
include('./includes/login_form.php');
}
?>

Answer 1

Session start must be the very first call in your script. https://www.w3schools.com/php/php_sessions.asp
<?php
session_start();
date_default_timezone_set("Europe/London");
require("db_connect.php");
etc...

Answer 2

Fixed the issue by adding an additional session_start(); directly at the beginning of the index.php file before any html. Seems strange that the login and session_check scripts wouldn't work without this as they also had session_start(); in them too.