I tried to make an easy login in my university task, but faced on with some troubles.
When I press the button all works fine including request, but redirect doesn’t work at all, I tried to use js instead, but even then it works so bad, one time works, and another doesn’t an don’t really understand why I didn’t find the solution.

PHP code:

<?php
ob_start();
include("index.html");
include("db_connection.php");

$select_users = "SELECT * FROM users";
try {
    $result = mysqli_query($conn, $select_users);
} catch (mysqli_sql_exception) {
    die("FAILED TO FETCH USERS");
}

function generate_insert_user($username, $pass)
{
    $hash = password_hash($pass, PASSWORD_DEFAULT);
    $insert_user = "INSERT INTO users (username, pass_hash) VALUES ('$username', '$hash');";
    return $insert_user;
}

if (isset($_POST['login'])) {
    if (mysqli_num_rows($result) > 0) {
        while ($row = mysqli_fetch_assoc($result)) {
            if (
                $row['username'] == trim(filter_input(INPUT_POST, 'username', FILTER_SANITIZE_SPECIAL_CHARS))
                && password_verify($_POST['password'], $row['pass_hash'])
            ) {
                header('Location: positions.php');
                exit();
            } else {
                unset($_POST['username']);
                unset($_POST['password']);
                die("FAILED TO LOGIN");
            }
        }
    }
}

if (isset($_POST['register'])) {
    if (is_string($_POST['username']) && is_string($_POST['password'])) {
        if (mysqli_num_rows($result) > 0) {
            $existed = false;
            while ($row = mysqli_fetch_assoc($result)) {
                if (
                    $row['username'] == trim(filter_input(INPUT_POST, 'username', FILTER_SANITIZE_SPECIAL_CHARS))
                ) {
                    $existed = true;
                }
            }
            if (!$existed) {
                $query = generate_insert_user(trim(filter_input(INPUT_POST, 'username', FILTER_SANITIZE_SPECIAL_CHARS)), trim(filter_input(INPUT_POST, 'password', FILTER_SANITIZE_SPECIAL_CHARS)));
                try {
                    mysqli_query($conn, $query);
                    header('Location: positions.php');
                    exit();
                } catch (mysqli_sql_exception) {
                    die("FAILED TO REGISTER");
                }
            } else {
                die("SUCH USER ALREADY EXIST");
            }
        }
    }
}

mysqli_close($conn);
ob_end_flush();