Html php contact form redirection - php

Question

so I been breaking my head over this, I am pretty new so sorry if I said something wrong. So I got hosting, installed wordpress on it, and I put my html on it, it cointains conctact form written in html and it has separate php file with process. It kindda works, if i test it, it will send me an email, but I put header('Location: ../front-page.php'); and I get white page with error line, I am pretty sure there is no error cuz well if you go to website, it uses that line and it works just fine, problem is only after submitting form. I am adding a code and putting php in Javascript. Also if possible some extra related questions, is it possible when it redirects to front page also add alert table with success message? And now when I get message, I do not see the one who wrote it email, and its pretty essential, also would be nice to fix . Thank you guys very much!
<?php
// define variables and set to empty values
$name_error = $email_error = $message_error = "";
$name = $email = $phone = $message = $url = $success = "";
//form is submitted with POST method
if ($_SERVER["REQUEST_METHOD"] == "POST") {
if (empty($_POST["name"])) {
$name_error = "Name is required";
} else {
$name = test_input($_POST["name"]);
// check if name only contains letters and whitespace
if (!preg_match("/^[a-zA-Z ]*$/",$name)) {
$name_error = "Only letters and white space allowed";
}
}
if (empty($_POST["email"])) {
$email_error = "Email is required";
} else {
$email = test_input($_POST["email"]);
// check if e-mail address is well-formed
if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
$email_error = "Invalid email format";
}
}
if (empty($_POST["message"])) {
$message_error = "Message is required";
} else {
$message = test_input($_POST["message"]);
}
if ($name_error == '' and $email_error == '' and $message_error == '' and $url_error == '' ){
$message_body = '';
unset($_POST['submit']);
foreach ($_POST as $key => $value){
$message_body .= "$key: $value\n";
}
$to = 'ignas.levinskas#mail.com';
$subject = 'Contact Form Submit';
if (mail($to, $subject, $message)){
header('Location: ../front-page.php');
$name = $email = $message = '';
}
}
}
function test_input($data) {
$data = trim($data);
$data = stripslashes($data);
$data = htmlspecialchars($data);
return $data;
}
<form id="contactform" method="post" action="http://li-designs.com/wp-content/themes/vcs-starter/assets/app.php" >
<input name="name" type="text" class="feedback-input" placeholder="Name" required/>
<span class="error"><?= $name_error ?></span>
<input name="email" type="text" class="feedback-input" placeholder="Email" required/>
<span class="error"><?= $email_error ?></span>
<textarea name="message" type="text" value="<?= $message ?>" class="feedback-input" placeholder="Message" ></textarea>
<span class="error"><?= $message_error ?></span>
<button name="submit" type="submit" id="contact-submit" data-submit="...Sending">Send</button>
</form>