<?php
include_once('_users.php');

// Set connected flag to false
$connected = false;
$form_sent = false;
// Check if POST data was send
if(isset($_POST['username']) && isset($_POST['pass'])) {
  $form_sent = true;

  $username = $_POST['username'];
  $password = $_POST['pass'];

  // Check if username exists in db and if passwords match
  if(isset($users[$username]) && $users[$username] == $password) {
    // If password match, set flag to true
    $connected = true;
  }
}
?>

<pre style="padding:10px; background: #eee"><?php var_dump($_POST); ?></pre>

<?php if(!$connected && $form_sent): ?>
<p><b style="color:red">Wrong password or username</b></p>
<?php endif; ?>

<?php if($connected): ?>
<div><b>You are connected</b></div>
<?php endif; ?>

<form method="POST">
  <input type="text" name="username" required/>
  <input type="password" name="pass" required/>
  <input type="submit" value="Validate"/>
</form>