Issue with simple PHP Contactform - php

Question

So I have some issues with my PHP Contactform on a HTML site.
Code below is what I have written in HTML. (that's not te problem)
<form action="sendmail.php" method="POST" class="contact-form">
<input type="text" placeholder="Amount" name="amount">
<input type="text" placeholder="Name" required name="name">
<input type="text" placeholder="Email Address" required name="email">
<div class="validation">
<button class="btn" name="submit">Send request</button>
// END HTML code. (BELOW is starts PHP).
$email_to = "domain#website.com";
$amount = $_POST["amount"];
$name = $_POST["name"];
$email = $_POST["email"];
$email_subject = "DOMAINNAME";
$headers = "From: " . $email . "\n";
$headers .= "Reply-To: " . $email . "\n";
$message = 'Name: ' . $name . ', email: ' . $email . ', amount: ' . $amount;
ini_set("sendmail", $email);
if (!filter_var($email, FILTER_VALIDATE_EMAIL) === false)
{$sent = mail($email_to, $email_subject, $message, $headers, "-f" .$email);}
{
header("Location:https://website.com");
} else {
echo "There has been an error sending your comments. Please try later.";
}
What do I wrong? I only wanna receive the --> name, amount and mail.
Only when I press submit on my HTML site, it's send me to website.com/sendmail.php (HTTP ERROR 500).
Thank you guys.