Problem with user authentication in PHP and variables and program flow - php

Question

I have been working for 2 days on a login and authentication system in PHP. For my logic it should be perfect, but I have problems as usual.
Here is the code:
<?php
ob_start();
session_start();
include "../navbar.php";
require_once '../database-connection.php';
include "../login.html";
if (isset($_SESSION['session_id'])) {
header('Location: ../index.php');
exit;
}
if (isset($_POST['login'])) {
$email = $_POST['email'] ?? '';
$password = $_POST['password'] ?? '';
$query = "
SELECT email, password
FROM users
WHERE email = :email
";
$check = $pdo->prepare($query);
$check->bindParam(':email', $email, PDO::PARAM_STR);
$check->execute();
$user = $check->fetch(PDO::FETCH_ASSOC);
if ($email == $user['email']) {
if (password_verify($password, $user['password'])) {
session_regenerate_id();
$_SESSION['session_id'] = session_id();
$sql = "
SELECT username, id FROM users
WHERE email = :email";
$check = $pdo->prepare($sql);
$check->bindParam(':email', $email);
$check->execute();
$user = $check->fetch();
$_SESSION['session_user'] = $user['username'];
$_SESSION['email'] = $email;
$_SESSION['user_id'] = $user['id'];
header('Location: ../index.php');
exit;
} else {
$msg = "Wrong Password";
}
} else {
$msg = "Wrong Email";
}
printf($msg, 'back');
}
The system works, it allows me to authenticate the users registered in the database, but I don't understand why the $msg variable that I use for errors is not printed.
Or better any echo that I put in any part of the listing is not printed.
I can't print anything anywhere. It looks like the program just hangs after the first few includes, yet it lets me authenticate.
(Yes guys I debug by printing the echo to understand up where the scripts work!)