Form action throws 'POST /mail.php Error (404): Not found' - php

Question

I want to send an email via html form with the help of php. However, it keeps
throwing same error every time claiming it couldn't find php file. I've tried
everything:
Installing NodeJs and running the code in localhost.
changing name="name" name="email" to something else.
Linking to another php script in case
something wrong with it.
Reading every SO page on this issue and trying every solution from there.
No luck at all.
Still POST /mail.php Error (404): "Not found". What am I missing here?
Can you make it work on your system?
HTML:
<form name="contactform" method="POST" action="mail.php">
<div>
<input type="text" name="user_name" placeholder="name" required />
<input type="email" name="user_email" placeholder="email" required />
<textarea type="text" name="comments" placeholder="message" required></textarea>
</div>
<button type="submit">SEND</button>
</form>
PHP:
<?php
$name = $_POST['user_name'];
$email = $_POST['user_email'];
$message = $_POST['comments'];
$formcontent=" From: $name \n Message: $message";
$recipient = "********#gmail.com";
$subject = "Contact Form";
$mailheader = "From: $email \r\n";
mail($recipient, $subject, $formcontent, $mailheader) or die("Error!");
echo "Thank You!";
?>

Answer 1

Use this :-
<form name="contactform" method="POST" action="mail.php">
<input type="text" name="user_name" id="user_name" placeholder="name" required />
<input type="email" name="user_email" id="user_email" placeholder="email" required />
<textarea type="text" name="comments" id="comments" placeholder="message" required></textarea>
<input type="submit" id="submit" value="SEND" />
</form>