viewstate MAC error - php

Question

I'm having trouble scraping an ASPX.NET website. Even after searching through stackoverflow I haven't been able to find a fix.
First off, this is a public accessible website that I'm trying to scrape so that I can email myself a copy occasionally in order to not bother having to go there with a browser.
My code goes so far as to capture the __VIEWSTATE and __EVENTVALIDATION but when I submit the form I get the "Validation of viewstate MAC failed" error.
Any ideas?
<?php
require_once ("simple_html_dom.php");
ini_set('display_errors', 'On');
error_reporting(E_ALL);
// Create curl connection
$url = 'http://www.ariautodirect.com/ui/index.aspx';
$cookieFile = 'cookie.txt';
$ch = curl_init();
// We must request the login page and get the ViewState and EventValidation hidden values
// and pass those along in the post request.
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
curl_setOpt($ch, CURLOPT_REFERER, 'http://www.ariautodirect.com/ui/index.aspx');
curl_setopt($ch, CURLOPT_HTTPHEADER,array('Origin: http://www.ariautodirect.com', 'Host: www.ariautodirect.com'));
curl_setopt($ch, CURLOPT_COOKIEJAR, $cookieFile);
curl_setopt($ch, CURLOPT_COOKIEFILE, $cookieFile);
$curl_scraped_page = curl_exec($ch);
// Grab ViewState and EventValidation data
$html = str_get_html($curl_scraped_page);
$viewState = $html->find("#__VIEWSTATE", 0);
$eventValidation = $html->find("#__EVENTVALIDATION", 0);
$previousPage = $html->find("#__PREVIOUSPAGE", 0);
//create array of data to be posted
// This matches exactly what I am seeing being posted when looking at Fiddler
$post_data['__EVENTTARGET'] = '';
$post_data['__EVENTARGUMENT'] = '';
$post_data['__VIEWSTATE'] = $viewState->value;
$post_data['__EVENTVALIDATION'] = $eventValidation->value;
$post_data['__PREVIOUSPAGE'] = 'Kn9nmdEGkbDIIX-v7Z_vg0t3njAt1lzkUm-uXH6djRcAGjPo6-w6RWKF3BBQz1ijEoJZGJVqOvHTqB5ghEU40C8xhnw1';//hard-coded because $previousPage->value throughs error
$post_data['ctl00$HeaderCtl1$LoginControl1$txtUserName'] = '';
$post_data['ctl00$HeaderCtl1$LoginControl1$txtPassword'] = '';
$post_data['ctl00$HeaderCtl1$LoginControl1$txtClientCode'] = '';
$post_data['ctl00$SearchCriteria1$drpDownMiles'] = '50';
$post_data['ctl00$SearchCriteria1$txtZipCode'] = '30301';
$post_data['ctl00$SearchCriteria1$ddlMake'] = 'All Makes ';
$post_data['ctl00$SearchCriteria1$CollapsiblePanelExtender1_ClientState'] = 'true';
$post_data['ctl00$SearchCriteria1$CollapsiblePanelExtender2_ClientState'] = 'true';
$post_data['ctl00$SearchCriteria1$CollapsiblePanelExtender3_ClientState'] = 'true';
$post_data['ctl00$SearchCriteria1$CollapsiblePanelExtender4_ClientState'] = 'true';
$post_data['ctl00$SearchCriteria1$CollapsiblePanelExtender5_ClientState'] = 'true';
$post_data['ctl00$SearchCriteria1$CollapsiblePanelExtender6_ClientState'] = 'true';
$post_data['ctl00$SearchCriteria1$CollapsiblePanelExtender7_ClientState'] = 'true';
$post_data['ctl00$SearchCriteria1$CollapsiblePanelExtender8_ClientState'] = 'true';
$post_data['ctl00$ContentPlaceHolder1$DataGridResults1$drpDownMiles'] = '50';
$post_data['ctl00$ContentPlaceHolder1$DataGridResults1$txtZipCode'] = '30301';
$post_data['ctl00$ContentPlaceHolder1$DataGridResults1$btnSearch'] = 'Search';
$post_data['ctl00$ContentPlaceHolder1$DataGridResults1$drpDownPageList'] = '100';
$post_data['ctl00$ContentPlaceHolder1$DataGridResults1$txtVinSearch'] = '';
$post_data['ctl00$ContentPlaceHolder1$DataGridResults1$SrtxtClientId'] = '';
$post_data['ctl00$ContentPlaceHolder1$DataGridResults1$SrtxtVehicleNo'] = '';
$post_data['ctl00$ContentPlaceHolder1$DataGridResults1$hdnSortYear'] = '';
$post_data['ctl00$ContentPlaceHolder1$DataGridResults1$hdnSortModel'] = '';
$post_data['ctl00$ContentPlaceHolder1$DataGridResults1$hdnSortMileage'] = '';
$post_data['ctl00$ContentPlaceHolder1$DataGridResults1$hdnSortPrice'] = '';
//traverse array and prepare data for posting (key1=value1)
foreach ( $post_data as $key => $value) {
$post_items[] = rawurlencode($key) . '=' . rawurlencode($value);
}
//create the final string to be posted using implode()
$post_string = implode ('&', $post_items);
$post_string = http_build_query($post_data);
//Set options for post
$urlAcctSummary = "http://www.ariautodirect.com/ui/DisplayResults.aspx";
curl_setopt($ch, CURLOPT_URL, $urlAcctSummary);
curl_setOpt($ch, CURLOPT_POST, TRUE);
curl_setopt($ch, CURLOPT_HTTPHEADER,array('Origin: http://www.ariautodirect.com', 'Host: www.ariautodirect.com', 'Content-Type: application/x-www-form-urlencoded'));
curl_setopt($ch, CURLOPT_POSTFIELDS, $post_string);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setOpt($ch, CURLOPT_REFERER, 'http://www.ariautodirect.com/ui/index.aspx');
// Perform our post request
$curl_scraped_page = curl_exec($ch);
echo $curl_scraped_page;
curl_close($ch);
?>