Get select value

How do I get the value of a select in the form, my form is just sending the other fields

PHP:

<?php
// Check for empty fields
if(empty($_POST['name']) || empty($_POST['email']) || empty($_POST['message']) || !filter_var($_POST['email'],FILTER_VALIDATE_EMAIL)){
echo json_encode(array('error'=>'true'));
return false;
}

$topic = $_post['topic'];
$name = $_POST['name'];
$email_address = $_POST['email'];
$message = $_POST['message'];

$lastname = $_POST['lastname'];
$phone = $_POST['phone'];
$subject = "Contato de formulário enviado via website";



// Create the email and send the message
$to = '[email protected]';// Add your email address inbetween the '' replacing [email protected] - This is where the form will send a message to.
$email_subject = $subject;
$email_body = "Você recebeu uma nova mensagem do site Dimasa Volkswagen.\n\n"."Tópico: $topic\n\nNome: $name\n\nEmail: $email_address\n\nPhone: $phone\n\nMessage: $message";
$headers = "From: [email protected]\n"; // This is the email address the generated message will be from. We recommend using something like [email protected].
$headers .= "Reply-To: $email_address"; 
mail($to,$email_subject,$email_body,$headers);
echo json_encode(array('success'=>'true'));
return true;            
?>

Script:

$(function() {
$("input,textarea").jqBootstrapValidation({
preventSubmit: true,
submitError: function($form, event, errors) {
    // additional error messages or events
},
submitSuccess: function($form, event) {
    event.preventDefault(); // prevent default submit behaviour
    // get values from FORM
    var topic = $("select#user-topic").val();
    var name = $("input#user-name").val();
    var email = $("input#user-email").val();
    var phone = $("input#user-phone").val();
    var message = $("textarea#user-message").val();
    var firstName = name; // For Success/Failure Message
    // Check for white space in name for Success/Fail message
    if (firstName.indexOf(' ') >= 0) {
        firstName = name.split(' ').slice(0, -1).join(' ');
    }
    $.ajax({
        url: "././mail/contact_me.php",
        type: "POST",
        dataType: 'json',
        data: {
            topic: topic,
            name: name,
            email: email,
            phone: phone,
            message: message
        },
        cache: false,
        success: function(data) {
            if(data.error){
                // Fail message
                $('#success').html("<div class='alert alert-danger'>");
                $('#success > .alert-danger').html("<button type='button' class='close' data-dismiss='alert' aria-hidden='true'>&times;").append("</button>");
                $('#success > .alert-danger').append("<span>Perdão " + firstName + ", parece que ocorreu uma falha no envio, tente novamente!</span>");
                $('#success > .alert-danger').append('</div>');
                //clear all fields
                $('#contactForm').trigger("reset");
            }
            else if(data.success){
                // Success message
                $('#success').html("<div class='alert alert-success'>");
                $('#success > .alert-success').html("<button type='button' class='close' data-dismiss='alert' aria-hidden='true'>&times;").append("</button>");
                $('#success > .alert-success').append("<span>Sua mensagem foi enviada com sucesso </span>");
                $('#success > .alert-success').append('</div>');
                //clear all fields
                $('#contactForm').trigger("reset");
            }
        }
    })
},
filter: function() {
    return $(this).is(":visible");
},
});
});


/*When clicking on Full hide fail/success boxes */
$('#name').focus(function() {
$('#success').html('');
});

Form HTML

<form id="contactForm" class="s-form wow zoomInUp" data-wow-delay="0.5s">
                        <div class="s-relative">
                            <select name="user-topic" id="user-topic" class="m-select">
                                    <option value="Not select">O que você precisa?</option>
                                    <option value="Topic 1">Solicitar contato com o vendedor</option>
                                    <option value="Topic 2">Agendar revisão</option>
                                    <option value="Topic 3">Cotação de peças</option>
                                    <option value="Topic 4">Crítica/Sugestão</option>
                                </select>
                            <span class="fa fa-caret-down"></span>
                        </div>
                        <input type="text" placeholder="NOME" value="" name="user-name" id="user-name" />
                        <input type="text" placeholder="E-MAIL" value="" name="user-email" id="user-email" />
                        <input type="text" placeholder="TELEFONE" value="" name="user-phone" id="user-phone" />
                        <textarea id="user-message" name="user-message" placeholder="DEIXE SUA MENSAGEM"></textarea>
                        <button type="submit" class="btn m-btn">Enviar mensagem<span class="fa fa-angle-right"></span></button>
                    </form>