refill_example.php

<?php
if(!empty($_POST)) 
{
    // This is the error message you will display if something goes wrong. For now this will be empty:
    $error = "";

    // Wrap everything inside a "try" block so that you can display 
    // the error message but still also render your form.
    try
    {
            // All of your error checking will be the same
            // you will just need to use `throw new Exception("error message")`
            // instead of `die("error message")`
	    if(empty($_POST['username'])) 
	    { 
	        throw new Exception("Please enter a username."); 
	    } 

	    if(empty($_POST['password'])) 
	    { 
	        throw new Exception("Please enter a password."); 
	    }

	    // You will need to include everything that should happen if all goes correctly
	    // within this "try" block, including the redirect back to the login page
	}
	catch (Exception $e)
	{
                // Set the error message to something you can display later:
		$error = $e->getMessage();
	}
}
?>

<!-- Display an error message like so -->

<?php if (!empty($error)): ?>
<div class="error">We ran into the following problem: <?php echo $error; ?></div>
<?php endif; ?>

<!-- You can refill your form inputs like so, adding a value based on the request -->

<input type="text" name="username" value="<?php echo $_POST['username']; ?>">

For http://stackoverflow.com/questions/23661672/how-to-have-this-form-be-validated-on-the-same-page