php - 使用 session 错误登录

Question

我已经使用与我的 sql 数据库的 session 创建了登录系统。当我输入正确的用户名和密码时，我可以登录。但是当我输入错误的时候，它并没有显示Incorrect username or password信息，而是停留在checklogin.php页面(空白页)。我该如何解决这个问题？这是 checklogin.php:

session_start();
$mypassword=$_POST['mypassword'];
$myusername=$_POST['myusername'];

    $dbh = new PDO("mysql:host=$hostname;dbname=...", $username, $password);
    $sql = "select * from user where username='$myusername' and password='$mypassword' ";
    foreach ($dbh->query($sql) as $row){

if (isset($myusername) && isset($mypassword)) {
  if (($row['username']==$myusername) && ($row['password']==$mypassword)) {
    $_SESSION['login']='true';
    echo "Login successfully";
    echo"<a href='log_out.php'>Logout</a>";
    }else{
    echo "Incorrect username or password";
    }
}
    }

Answer 1

$row=$dbh->query($sql);
if ($row['username']='' && $row['password']!='') {
  if (($row['username']==$myusername) && ($row['password']==$mypassword)) {
    $_SESSION['login']='true';
    echo "Login successfully";
    echo"<a href='log_out.php'>Logout</a>";
    }else{
    echo "Incorrect username or password";
    }
}