为什么PHP MySQL INSERT通过AJAX调用只是偶尔发生

我有一个带有模态的页面。模态代码在其自己的PHP文件中，名为"tour-moder.PHP"：

<div class="modal fade" id="tourModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
    <div class="modal-dialog">
        <div class="modal-content">
            <div class="modal-header">
                <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
            </div>
            <div class="modal-body">
                <div class="errors2"></div>
                <form class="tour-card" action="insert.php" method="post">
                    <div class="form-group col-sm-12">
                        <input type="text" class="form-control" id="firstNameTourCard" placeholder="First Name"/>
                        <input id="middleInitialTour" type="hidden" value="">
                        <input type="text" class="form-control" id="lastNameTourCard" placeholder="Last Name"/>
                        <input type="email" class="form-control" id="emailTourCard"/>
                        <input type="tel" class="form-control" id="phoneTourCard"/>
                        <select class="form-control" id="tourDate">
                            <option>May 25 2016</option>
                            <option>May 28 2016</option>
                        </select>
                        <select class="form-control" id="tourTime">
                            <option>1:00 PM</option>
                            <option>1:30 PM</option>
                            <option>2:00 PM</option>
                        </select>
                    </div>  
                    <input id="tour-btn" type="button" value="SUBMIT" class="btn btn-default tour-button"/>
                    <button type="button" class="btn btn-default tour-button" data-dismiss="modal">CLOSE</button>
                </form>
            </div>
        </div>
    </div>
</div>

在同一个文件上，我有一个jQuery/AAJAX脚本，如下所示：

<script>
    jQuery("#tour-btn").click(function (e) {
        var str = "firstNameTourCard=" + jQuery("#firstNameTourCard").val()
            + "&lastNameTourCard=" + jQuery("#lastNameTourCard").val()  
            + "&emailTourCard=" + jQuery("#emailTourCard").val()
            + "&phoneTourCard=" + jQuery("#phoneTourCard").val()
            + "&tourDate=" + jQuery("#tourDate").val()
            + "&tourTime=" + jQuery("#tourTime").val()
            + "&middleInitialTour=" + jQuery("#middleInitialTour").val();
        jQuery.ajax({
            type: "post",
            url: "/wp-content/themes/myTheme/tour-form.php",
            data: str,
            dataType: "json",
            success: function(result) {
                if (result.success == 1) {
                    console.log ('success');
                    jQuery('#tourModal').modal('hide');
                } else {
                    console.log ('failure');
                    jQuery(".errors2").html(result.errors2);
                }
            }
        });
    });
</script>

最后，我有一个在AJAX调用中引用的"tour-form.php"文件：

<?php
try {
    $db = new PDO('mysql:host=localhost;dbname=MYDBNAME;charset=utf8',
                  'USERNAME',
                  'PASSWORD');
    $db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_SILENT);
    $db->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
}
catch(PDOException $ex) {
    echo "did not connect...";
}

$firstNameTourCard = trim(isset($_POST['firstNameTourCard']) ? $_POST['firstNameTourCard'] : '');
$lastNameTourCard = trim(isset($_POST['lastNameTourCard']) ? $_POST['lastNameTourCard'] : '');
$emailTourCard = trim(isset($_POST['emailTourCard']) ? $_POST['emailTourCard'] : '');
$phoneTourCard = trim(isset($_POST['phoneTourCard']) ? $_POST['phoneTourCard'] : '');
$tourDate = trim(isset($_POST['tourDate']) ? $_POST['tourDate'] : '');
$tourTime = trim(isset($_POST['tourTime']) ? $_POST['tourTime'] : '');
if ($_POST['middleInitialTour'] != '') {
    $result = array(
    "success" => 1);
    die(json_encode($result));
}

$errors2 = Array();
if (sizeof($_POST) > 0) {
    if ($emailTourCard === '' || !filter_var($emailTourCard, FILTER_VALIDATE_EMAIL)) {
        $errors2[] = '<span style="color:red;"> Please enter a valid email address. </span>';
    }
}
if (sizeof($errors2) > 0 || sizeof($_POST) == 0) {
    $result = array(
    "errors2" => implode("<br/>", $errors2),
    "success" => 0);
    die(json_encode($result));
}
$randID2 = md5(uniqid(rand(), true));
$sql="INSERT INTO tours (id, first_name, last_name, email, phone, tour_date, tour_time, signup_date)
      VALUES
      ('".$randID2."', '".$firstNameTourCard."', '".$lastNameTourCard."', '".$emailTourCard."', '".$phoneTourCard."', '".$tourDate."', '".$tourTime."', NOW())";
$sth = $db->prepare($sql);
$sth->execute();

$result = array(
    "success" => 1);
die(json_encode($result));
?>

问题是，在我尝试这种方法的5次中，只有1次SQL查询实际执行。AJAX调用被调用，PHP文件发送成功消息，模态关闭，但INSERT语句很少实际工作。有人看到什么不对吗？