Trying to organize all my code into classes, and I can't get the database queries to work inside a class. I tested it without the class wrapper, and it worked fine. Inside the class = no dice. What about my classes is messing this up?

class ac
  {
  public function dbConnect() 
    {
    global $dbcon;

    $dbInfo['server'] = "localhost";
    $dbInfo['database'] = "sn";
    $dbInfo['username'] = "sn";
    $dbInfo['password'] = "password"; 

    $con = "mysql:host=" . $dbInfo['server'] . "; dbname=" . $dbInfo['database'];
    $dbcon = new PDO($con, $dbInfo['username'], $dbInfo['password']);
    $dbcon->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $error = $dbcon->errorInfo();

    if($error[0] != "") 
      {
      print "<p>DATABASE CONNECTION ERROR:</p>";
      print_r($error);
      }
    }


  public function authentication()
    {
    global $dbcon;

    $plain_username = $_POST['username'];
    $md5_password = md5($_POST['password']);

    $ac = new ac();
    if (is_int($ac->check_credentials($plain_username, $md5_password)))
      {
      ?>
      <p>Welcome!</p> <!--go to account manager here-->
      <?php
      }
    else
      {
      ?>
      <p>Not a valid username and/or password. Please try again.</p>
      <?php
      unset($_POST['username']);
      unset($_POST['password']);
      $ui = new ui();
      $ui->start();
      }
    }



  private function check_credentials($plain_username, $md5_password)
    {
    global $dbcon;

    $userid = $dbcon->prepare('SELECT id FROM users WHERE username = :username AND password = :password LIMIT 1');
    $userid->bindParam(':username', $plain_username);
    $userid->bindParam(':password', $md5_password);
    $userid->execute();

    print_r($dbcon->errorInfo());

    $id = $userid->fetch();
    Return $id;
    }
  }

And if it's any help, here's the class that's calling it:

require_once("ac/acclass.php");
$ac = new ac();
$ac->dbconnect();
class ui
  {
  public function start()
    {
    if ((!isset($_POST['username'])) && (!isset($_POST['password'])))
      {
      $ui = new ui();
      $ui->loginform();
      }
    else
      {
      $ac = new ac();
      $ac->authentication();
      }
    }

  private function loginform()
    {
    ?>
    <form id="userlogin" action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post">
     User:<input type="text" name="username"/><br/>
     Password:<input type="password" name="password"/><br/>
     <input type="submit" value="submit"/>
    </form>
    <?php
    }
  }