if (isset($_POST['submit']))
{
    $user = $_POST['user'];
    $pass = $_POST['pass'];
    $admin = $_POST['admin'];

    if( $user == "" || $pass == "")
    {
        echo '<div id ="errormsg">Please fill in all fields</div>';
    }

    else
    {
        $query = mysqli_query($dbcon, "SELECT * FROM users WHERE username = '$user'
        and password = '$pass' and admin = '$admin' ") or die ("Can't query the database");
        $count = mysqli_num_rows($query);

        if($count == 1)
        {
            if ($admin == 1)
            {
                $_SESSION['username'] = $user;
                header("location: admin.php");
            }
            else if ($admin == 0)
            {
                $_SESSION['username'] = $user;
                header("location: users.php");
            }
            else
            {
                echo '<div id="errormsg">No matches, try again</div>';
            }
        }
    }
}

<form action="<?=$_SERVER['PHP_SELF']?>" method="post">
  <fieldset>
    <label class="login">Username:</label><input type="text" name="user" /><br />
    <label class="login">Password:</label><input type="password" name="pass" /><br />
    <label class="login">Admin?:</label><input type="checkbox" value="1" name="admin" /><br/>

    <input type="submit" name="submit" value="Login" />
  </fieldset>
</form

if ($admin == 1)

if (isset($admin))

if (isset($admin))
{
    $_SESSION['username'] = $user;
    header("location: admin.php");
    exit;
} else {
    $_SESSION['username'] = $user;
    header("location: users.php");
    exit;
}

$query = mysqli_query($dbcon, "SELECT * FROM users WHERE username = '".$user."'' and password = '".$pass."''") or die ("Can't query the database");