function accounts_init(): void {
  if (!isset($_SESSION['accounts'])) $_SESSION['accounts'] = [];
  if (!isset($_SESSION['active_account'])) $_SESSION['active_account'] = null;
}

function set_active_account(string $email): void {
  accounts_init();
  $_SESSION['active_account'] = $email;
}

function get_active_account(): array {
  accounts_init();
  $active = $_SESSION['active_account'];
  if(!$active || empty($_SESSION['accounts'][$active])) return ['', ''];

  $a = $_SESSION['accounts'][$active];
  $email = (string)($a['email'] ?? '');
  $pass  = function_exists('dec') ? dec((string)($a['pass'] ?? '')) : (string)($a['pass'] ?? '');
  return [$email, $pass];
}

function require_login(): void {
  [$email, $pass] = get_active_account();
  if($email === '' || $pass === ''){
    header('Location: login.php');
    exit;
  }
}
<br />
<b>Fatal error</b>:  Uncaught Error: Call to undefined function accounts_init() in C:\inetpub\vhosts\jner-gaming.de\mail.Jner-Gaming.de\index.php:4
Stack trace:
#0 {main}
  thrown in <b>C:\inetpub\vhosts\jner-gaming.de\mail.Jner-Gaming.de\index.php</b> on line <b>4</b><br />