<form method="post" action="php/login.php">
    <div class="form-group">
      <input class="form-control" type="text" name="username" placeholder="Username" required>
    </div>

    <div class="form-group">
      <input class="form-control" type="password" name="password" placeholder="Password" required>
    </div>

    <div class="form-group">
      <input class="btn btn-primary" type="submit" name="login" value="Login">
    </div>
  </form>

<?php
require_once "../functions.php";

db_connect();

$sql = "SELECT id, username, password FROM users WHERE username = ?";
$statement = $conn->prepare($sql);
$statement->bind_param('s', $_POST['username']);
$statement->execute();
$statement->store_result();
$statement->bind_result($id, $username, $password);
$statement->fetch();


if ($statement->execute()) {
if(password_verify($_POST['password'], $password)) {
  $_SESSION['user_id'] = $id;
  $_SESSION['user_username'] = $username;
  redirect_to("/home.php");
 } else {
  redirect_to("../index.php?login_error=true");
 }
 } else {
echo "Error: " . $conn->error;
 }

<?php
session_start();

function db_connect() {
global $conn; // db connection variable
$db_server = "localhost";
$username = "root";
$password = "xxxxxxxxx";
$db_name = "xxxxxxxxxx";

// create a connection
$conn = new mysqli($db_server, $username, $password, $db_name);

// check connection for errors
if ($conn->connect_error) {
  die("Error: " . $conn->connect_error);

}

}

function redirect_to($url) {
header("Location: " . $url);
exit();
}

function is_auth() {
return isset($_SESSION['user_id']);
}

function check_auth() {
if(!is_auth()) {
  redirect_to("../index.php?logged_in=false");
}
}

if ($statement->execute()) {
    if (password_verify($_POST['password'], $password)) {
        $_SESSION['user_id'] = $id;
        $_SESSION['user_username'] = $username;
        redirect_to("/home.php");
    } else {
        redirect_to("../index.php?login_error=true");
    }
} else {
    echo "Error: " . $conn->error;
}

if (password_verify($_POST['password'], $password)) {