in two way you can do this!
-1:
use like that parameter in URL
header('Location: loginpage.php?error=1');
in loginpage.php in JavaScript section check URL is contain error or not!
look at this link
just replace "q" with error!
make a function and check if contain error
show a error message!
<script>
function logincheck(){
// code
}
if(logincheck()){
// do something
//alert('error');
// or append a error in html element
}
</script>
you can check this query string in php but JavaScript way is better
-2
use session
after validate set a session with error message
like this:
$_SESSION['login_error'] = 'error in login!';
and in loginpage.php
check if that session have a value, show that to user,and unset session
like this:
if(isset($_SESSION['login_error'])){
echo '<p class="error">Error Logging In!</p>';
unset($_SESSION['login_error']);
}