Ajax表单不工作(jQuery-PHP)

查询:

$("#contact-us-form").submit(function () {
    var nameVal = $("input[name=name]").val();
    var companyVal = $("input[name=company]").val();
    var titleVal = $("input[name=title]").val();
    var emailVal = $("input[name=email]").val();
    var phoneVal = $("input[name=phone]").val();
    var messageVal = $("input[name=message]").val();
    $.post("mailer.php", {
        name: nameVal,
        company: companyVal,
        title: titleVal,
        email: emailVal,
        phone: phoneVal,
        message: messageVal
    }, function (data) {
        alert("Data Loaded: " + data);
        $('#thanks').show();
    });
    return false;
});

<?php

if(isset($_POST['submit'])) {

    $to = "admin@domain.com";
    $subject = "Inquiry";
    $name = $_POST['name'];
    $company = $_POST['company'];
    $title = $_POST['title'];
    $email = $_POST['email'];
    $phone = $_POST['phone'];
    $message = $_POST['message'];

    $body = <<<HEREDOC
        From: $name
        Company: $company
        Title: $title 
        E-Mail: $email
        Phone: $phone \n
        Message: $message 
HEREDOC;

    echo 'success';
    mail($to, $subject, $body);

} else {
    echo "failure";
}

?>

页面上的数据警报返回 failure ,我不明白为什么!