HTML+PHP form not sending to my email - php

Question

Lots of similar questions have been asked, but couldn't find an answer to the exact problem I have. I have a form that works completely fine, but I asked someone to design it nicer (which he did) and now it just doesn't work anymore. I don't know what's causing it.
I'm assuming there is something wrong in my HTML code for the form, since the .php page works perfectly fine (I copy/pasted it entirely) on my previous page.
Any idea?
<form name="htmlform" method="post" action="html_form_send.php">
<div class="row has-form">
<div class="col-xs-12 col-sm-8 col-sm-offset-2">
<div class="form-group">
<label for="first_name" class="col-sm-3 control-label">Prénom *</label>
<div class="col-sm-9">
<input type="text" class="form-control" id="first_name">
</div>
</div>
<div class="form-group">
<label for="last_name" class="col-sm-3 control-label">Nom *</label>
<div class="col-sm-9">
<input type="text" class="form-control" id="last_name">
</div>
</div>
<div class="form-group">
<label for="email" class="col-sm-3 control-label">Email *</label>
<div class="col-sm-9">
<input type="email" class="form-control" id="email">
</div>
</div>
<div class="form-group">
<label for="telephone" class="col-sm-3 control-label">Téléphone</label>
<div class="col-sm-9">
<input type="text" class="form-control" id="telephone">
</div>
</div>
<div class="form-group">
<label for="comments" class="col-sm-3 control-label">Votre critique *</label>
<div class="col-sm-9">
<textarea class="form-control" rows="6" id="comments"></textarea>
</div>
</div>
<div class="col-xs-12 col-sm-9 col-sm-offset-3 text-center">
<button type="submit" class="btn btn-primary btn-lg btn-block">Envoyer</button>
</div>
</div>
</div>
</form>

Answer 1

your inputs have no "name" attribute add a "name" attribute with the same value as the "id" attribute to each input tag as
Php uses NAME as the identifier when posted.
To Clearify:
if you try and get an input value from the POST in PHP you have to use the following code:
$var = $_POST['IDENTIFIER'];
Where IDENTIFIER is the value of the "name" attribute of the INPUT or TEXTAREA or any other HTML FORM element you're trying to retrieve.

Answer 2

all your inputs need attribute name="first_name" and etc:
<input type="text" class="form-control" id="first_name" name="first_name">
html_form_send.php must looks like:
<?php
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
$first_name = $_POST['first_name'];
$last_name = $_POST['last_name'];
$email = $_POST['email'];
$telephone = $_POST['telephone'];
$comments = $_POST['comments'];
//preparing mail data
$from = $email;
$to = "yourmail#exemple.com";
$subject = "Sending Form From site";
$message = "First name: $first_name \r\nLast name:$last_name \r\nTelephone: $telephone \r\nComments: $comments";
$headers = 'From: Birthday Reminder <birthday#example.com>' . "\r\n";
//sending
mail($to, $subject, $message, $headers);
}
?>