How to prevent Gmail from stacking into a "conversation" emails sent via PHP mail()?

Question

I have a web form which uses the following PHP code to send email:

$senderEmail = "sender@sender.com"
$senderName = "John Smith"

$noreplyEmail = "noreply@receiver.com"
$receiverEmail = "inbox@receiver.com"

$header = "MIME-Version: 1.0\r\nContent-type: text/plain; charset=UTF-8\r\n";
$header .= 'From: "' . $senderName . '" <' . $noreplyEmail . ">\r\n";
$header .= 'Reply-To: "' . $senderName . '" <' . $senderEmail . ">";

$subject = "Contact form";
$message = "...";

mail($receiverEmail, $subject, $message, $header);

The problem is that although each time $senderName, $senderEmail and $message are different in the inbox of the receiver (which is a Gmail domain inbox) the emails get stacked into a conversation by the Gmail's system.

What would be the proper way to prevent this stacking and receive them always as individual separate emails?

Answer 1

Easy: You have to change the Subject of your email:

mail('hi@example.com', 'Test', 'Hi there 3')

mail('hi@example.com', 'Test 3', 'Hi there 3')

Maybe something like $subject = "Contact from" . $senderName;