PHP Mail - Sanitizing and Validating for Basic Form

Question

I'm looking for the safest and most effective way to use php mail for a basic form.

I have the below code, and it seems to work decently, but I'm very new to php and would like input on if there's better ways to execute this. Should I be doing more to help protect from injection? Or do I even need to worry about that with plain text email?

Right now I'm sanitizing the email field. Should I sanitize all fields? I figured out how to validate the email too, but the problem with that is I don't like it if a legit user writes a super long message and accidentally inputs a mistake in his email, then php gives a non-valid email error and wipes away the whole message. I'd rather just sanitize it and the message still gets sent. I do have Jquery validate in place so that helps too.

I'm open to any advice on how to improve this form, or if you see anything I did wrong, please let me know! Thank you.

<?php
if (isset($_POST['submit'])) {


$send_to = "receiver@domain.com";
$subject = "Contact Page Form";
$headers = 
'From: "Sender Name" <sender@domain.com>' . "\r\n" .
'Reply-To: ' . $_POST['name'] . '<' . $_POST['email'] . '>' . "\r\n" .
'Content-type: text/plain; charset=iso-8859-1';
'X-Mailer: PHP/' . phpversion();
$success = "<p class=\"formsuccess\">Thank you. We received your message!</p>";


$name = $_POST['name'];
$email = filter_var($_POST['email'], FILTER_SANITIZE_EMAIL);
$phone = $_POST['phone'];
$message = $_POST['message'];

$content = "
Name: $name
Email: $email
Phone: $phone

Message: $message
";

mail($send_to,$subject,$content,$headers);
echo $success;

} else {
?>
<form id="contactform" method="post">

NAME: <input type="text" name="name" required>
EMAIL: <input type="email" name="email" required>
PHONE: <input type="tel" name="phone" required>
MESSAGE:<textarea name="message" required></textarea>

<button type="submit" name="submit">Submit</button>

</form>
<script>
$("#contactform").validate();
</script>
<?php } ?>

Answer 1

If validation fails on the server side, you just need to repopulate the form with submitted values:

NAME: <input type="text" name="name" value="<?php echo $name; ?>" required>

Set the variables to default values before your submission check to avoid 'undefined variable' errors.

Next, if you're worried about validity, you should avoid calling mail() directly - your script as it stands is vulnerable to header injection attacks and all kinds of corruption. Use a library like PHPMailer (which you already tagged the question with) to do it correctly and follow the examples provided for processing forms.