Ajax返回错误500，无论给什么文件作为url

我得到一个服务器500错误与下面的代码。我还用测试文件测试了它，仍然有500个错误。文件设置为644。

这是Ajax代码

function paynlSubmitPayment(userid ,credits ,profile ,bankId) {
paynlSubmitForm = false;
var newwin = paynlPaymentPopup('/Paynl/paynl-loading.html');
var product = jQuery("input[name='selectedProduct']").val();
$.ajax({
    type: "POST",
    url: "/Paynl/paynl-api.php",
    contentType: "application/json; charset=utf-8",
    dataType: "json",
    data: {
        mode:'startPayment',
        product:product,
        profile:profile,
        idealBank:bankId,
        userid:userid,
        credits:credits
    }, 
    success: function(data) {
        if('TRUE' == data.result) {
            newwin.location.href = data.issuerUrl;
            newwin.focus();
        } else {
            newwin.close();
            alert(data.error);
        }
    },
    error: function(){
        newwin.close();
        alert('Fail');
    },
    complete: function() {
        paynlSubmitForm = true;
    }
});
} 
function paynlCheckFormSubmit() {
return paynlSubmitForm;
}

加载弹出工作，但它显示了错误'失败'部分。我也试过把整个url与http，但仍然没有成功。我可以张贴php文件，但似乎错误是在Ajax的某个地方，因为测试文件也没有工作。

<?php
# Include classes
include realpath(dirname(__FILE__)) . '/Paynl/Paynl.php';
require_once($_SERVER['DOCUMENT_ROOT'].'/includes/connect.php');
# Controleer of installer is doorlopen
if (!class_exists('PaynlConfigPPT') || !class_exists('PaynlConfigPPC')) {
trigger_error('Run installer first!', E_USER_ERROR);
die();
}

switch (strtoupper($_POST['mode'])) {
case 'STARTPAYMENT':
    try {
        # PPT transactie starten
        $paynlPPT = new PaynlPPT();
        $products = $paynlPPT->getProducts();
        # Bepaal bedrag aan de hand van de producten in de
        # config en het ID van het geselecteerde product
        $transactionValue = $products[$_POST['product']]['price'];
        # Bij ideal wordt een bank verwacht
        if (array_key_exists('idealBank', $_POST)) {
            if ((int) $_POST['idealBank'] <= 0) {
                # geen bank geselecteerd voor doorsturen naar internetbankieren.
                throw new PaynlException('Geen bank geselecteerd');
            }
            $idealBankId = (int) $_POST['idealBank'];
            $selectedProfile = PaynlPPT::PROFILE_ID_IDEAL;
        } else {
            # Geen ideal, dus bankId=null
            $idealBankId = null;
            $selectedProfile = (int) $_POST['profile'];
        }

        # Transactie submitten bij Pay.nl
        $paynlPPT->setProductId($_POST['product']);
        $data = $paynlPPT->startTransaction($transactionValue, $selectedProfile, $idealBankId);
        // CG - Insert Transaction Details
        mysql_query("INSERT INTO transactions(transid, amount, userid, pakket, credits, betaald, uts) VALUES('".$data['orderId']."','" . $transactionValue . "', '".$_POST['userid']."', '" . $_POST['product'] . "', '".$_POST['credits']."', 0, " . time() . ");") or die(mysql_error());
        // CG - Insert Transaction Details
    } catch (PaynlException $e) {
        $data = array(
            'result' => 'FALSE',
            'error' => $e->getMessage()
        );
    }
    echo json_encode($data);
    break;
}