内部回声功能如果功能不起作用

Question

I have this really annoying problem with showing up a username if the user is logged from the previous PHP file.

In a navbar, I want to show his username if he's registered or show this "Přihlásit se" link ("log in" in Czech).

So, I created the source.txt file, which is like a base code for the environment of the website (I always want that navbar, where a user can see if he's logged or not and some other stuff)

First file source.txt

   <div class="container">
    <ul class="nav-main">

<?php

  if (isset($user_id)) {

    echo ("<li><a>$user_name</a></li>");
  }

  else {

    echo ("<li><a>Přihlásit se</a></li>");
  }

 ?>

Second file index.php

<?php

  if (isset($_POST['user_id'])) {

  $user_id = $_POST['user_id'];
  $user_name = $_POST['user_name'];

}

  readfile ("html/source.txt");

 ?>

I expect the output "Přihlásit se" link, because I haven't set the $user_id with POST method.

But actual output is just this:

$user_name"); } else { echo ("
Přihlásit se
"); } ?>