I have created a form for visitors of my website can message me if they want but for some reason it won't send the message and I do not know how to debug it.
Form:
<form method="post" action="sendmail.php">
                                <fieldset>
                                    <div class="form-group">
                                        <label for="firstname" class="sr-only">First Name</label>
                                        <input type="text" name="firstname" class="form-control" placeholder="First Name" id="firstname">
                                    </div>
                                    <div class="form-group">
                                        <label for="lastname" class="sr-only">Last Name</label>
                                        <input type="text" name="lastname" class="form-control" placeholder="Last Name" id="lastname">
                                    </div>
                                    <div class="form-group">
                                        <label for="email" class="sr-only">Email</label>
                                        <input type="text" name="email" class="form-control" placeholder="Email" id="email">
                                    </div>
                                    <div class="form-group">
                                        <label for="message" class="sr-only">Message</label>
                                        <textarea class="form-control" placeholder="Message" id="message"></textarea>
                                    </div>
                                    <input type="submit" class="btn btn-primary" value="Send Message" >
                                </fieldset>
                            </form>
and the sendmail.php for the info to be sent:
    <?php 
if(isset($_POST['submit'])){
    $to = "email@email.tld"; 
    $from = $_POST['email']; 
    $first_name = $_POST['firstname'];
    $last_name = $_POST['lastname'];
    $message = $firstname . " " . $last_name . " wrote the following:" . "\n\n" . $_POST['message'];
    $message2 = "Here is a copy of your message " . $firstname . "\n\n" . $_POST['message'];
    $headers = "From:" . $from;
    $headers2 = "From:" . $to;
    mail($to,$message,$headers);
    mail($from,$message2,$headers2); // sends a copy of the message to the sender
    echo "Mail Sent. Thank you " . $first_name . ", we will contact you shortly.";
    }
?>
 
    