Php AJAX验证&；在表单指向Paypal之前将数据发送到DB

我想在表单发送到Paypal之前使用AJAX和更新DB创建验证

<form method="post" action="https://www.sandbox.paypal.com/cgi-bin/webscr" class="paypal-button" target="_top">
    <div class="hide" id="errorBox"></div>
    <input type="hidden" name="env" value="www.sandbox"><input type="hidden" name="notify_url" value="www.immortalinsurance.com">
    <input type="hidden" name="tax" value="0">
    <input type="hidden" name="shipping" value="0">
    <input type="hidden" name="currency_code" value="AUD"><input type="hidden" name="amount" value="$15.00">
    <input type="hidden" name="quantity" value="1">
    <input type="hidden" name="item_name" value="Cover Yourself &amp; a Friend"><input type="hidden" name="custom" value="">
    <p class="paypal-group">
        <label class="paypal-label" for="policy_name1">policy_name1
            <input type="text" class="paypal-input" name="policy_name1">
        </label></p>
    <p class="paypal-group">
        <label class="paypal-label" for="policy_name2">policy_name2
            <input type="text" class="paypal-input" name="policy_name2">
        </label></p>
    <input type="hidden" name="button" value="buynow"><input type="hidden" name="cmd" value="_xclick">
    <input type="hidden" name="business" value="xxx@gmail.com"><input type="hidden" name="bn" value="JavaScriptButton_buynow">
    <button type="submit" class="paypal-button large">Buy Now</button>    
</form>

这是我的initialstoring.php：

<?php

include("../application/config/db_config.php");

$itemid = $_POST['itemid'];
$date = date('d-m-Y h:i:s');
$itemprice = $_POST['itemprice'];

$sql = "SELECT * FROM policies WHERE id = $itemid";
$stmt = $mysqli->query($sql);
$row = mysqli_fetch_assoc($stmt);


if ($itemprice == $row['price']) {
    $insert_row = $mysqli->query("INSERT INTO purchases (name_on_policy, other_on_policy, dateofpurchase, status) VALUES ('$certname', '$othercertname', '$date', '$status')");
    $lastid = $mysqli->insert_id;
} else {
    echo "error";
}

?>

<?php

include("../application/config/db_config.php");

$itemid = $_POST['itemid'];
$date = date('d-m-Y h:i:s');
$itemprice = $_POST['itemprice'];

$sql = "SELECT * FROM policies WHERE id = $itemid";
$stmt = $mysqli->query($sql);
$row = mysqli_fetch_assoc($stmt);


if ($itemprice == $row['price']) {
    $insert_row = $mysqli->query("INSERT INTO purchases (name_on_policy, other_on_policy, dateofpurchase, status) VALUES ('$certname', '$othercertname', '$date', '$status')");
    $lastid = $mysqli->insert_id;
} else {
    echo "error";
}

?>