How to show error messages in HTML page in PHP?

I would prefer Jquery Validation or Ajax based Authentication. But still you can do it this way:

Put your Error Message in Session like this :

$_SESSION['Error'] = "You left one or more of the required fields.";

Than simple show it like this:

if( isset($_SESSION['Error']) )
{
        echo $_SESSION['Error'];

        unset($_SESSION['Error']);

}

In this case you can assign multiple messages in different Operations.

header("Location:http://localhost/login.php?x=1")

In the login.php

if(isset($_GET('x'))){
//your html for error message
}

Hope it helps you,

In processlogin.php,

        if(!$_POST["username"] || !$_POST["password"])
            {
                $msg = "You left one or more of the required fields.";
                $msgEncoded = base64_encode($msg);
                header("location:login.php?msg=".$msgEncoded);
            }

in login.php file,

          $msg = base64_decode($_GET['msg']);
            if(isset($_GET['msg'])){

                if($msg!=""){
                    echo $msg;
                }
            }