Is it possible to authenticate a user on multiple services?

Question

I have two distinct services from each other, but both share the same data for authentication, so I made a single-screen entry that gives you the service options that the end user wants to enter. Upon entering it will pass a validation and gain a session name that will be retrieved on the home page of the chosen service.

In the index of each service I recover the session variable, to recover this data I use the server variable, more specifically $_SERVER['HTTP_REFERER'] where I retrieve the last page to retrieve the session itself.

Basically it is a gambiarra that works up to a certain point, because when refreshing the page, the server variable is empty, for not having a concrete reference from where it came out, so I can not use it to authenticate the user. With my current knowledge I see no other way to do this authentication on both services.

I would like to understand a better way of doing this authentication, whether or not I should continue doing so, any opinion is valid.

For ease of understanding, I have made available in blocks each page with brief code to give cohesion to everything I explained above.

AUTHENTICATION PAGE

  $some_name = session_name("some_name");
  session_set_cookie_params(360, '/', '.digiapp.com.br');
  if (!isset($_SESSION)) session_start();

  $_SESSION['userId'] = $row_1->id_diretora;
  $_SESSION['userNome'] = $row_1->nome_diretora;
  $_SESSION['userEmail'] = $row->email_diretora;
  $_SESSION['userPass'] = $row->pass_biblioteca_diretora;

SERVICE PAGE 1

if($_SERVER['HTTP_REFERER'] == 'http://login.digiapp.com.br/' || $_SERVER['HTTP_REFERER'] == 'http://login.digiapp.com.br/index.php'){
    $some_name = session_name("some_name");
    session_set_cookie_params(360, '/', '.digiapp.com.br');
    session_start();
}else{
    // Destrói a sessão por segurança
    session_destroy();
    // Redireciona o visitante de volta pro login
    header("Location: ../index2.php"); exit;
}