PHP error display - php

Question

I am new with php, but I have already made a registration script that works fine. But the problem is every time I press the submit button to check my error, I'm going to a new page.
My question is how I make that error comes on the same page?
The code I am useing for the html form.
I want the error display in the error div box that I made Any idea ?
<div id="RegistrationFormLayout">
<h1>Registration Page</h1>
<div id="ErrorMessage"></div>
<form action="script/registration.php" method="post">
<label for="Username">Username</label>
<input type="text" name="Regi_username">
<label for="FirstName">FirstName</label>
<input type="text" name="Regi_Firstname">
<label for="LastName">LastName</label>
<input type="text" name="Regi_Lastname">
<label for="EamilAddress">Regi_EmailAddres</label>
<input type="text" name="Regi_EmailAddres">
<label for="Password">Password</label>
<input type="password" name="Regi_password">
<button type="submit" value="Submit" class="Login_button">Login</button>
</form>
</div>

Answer 1

If I understand correctly, you want form validation errors there. This is a very common pattern, and the simple solution is to always set a form's action attribute to the same page that displays the form. This allows you to do the form processing before trying to display the form (if there are $_POST values). If the validation is successful, send a redirect header to the "next step" page (with header()).
The basic pattern looks like this (in very very simplified PHP)
<?php
if(count($_POST)) {
$errors = array();
$username = trim($_POST['Regi_username']);
if(empty($username)) {
$errors[] = 'username is required';
}
if(count($errors) == 0) {
header('Location: success.php');
die();
}
}
<ul class="errors">
<?php foreach($errors as $error) { ?>
<li><?php echo $error;?></li>
<?php } ?>
</ul>