PHP and CURL to scrap data

Dear All,
I want to download lots of papers from one site. Along with that, I need to create csv file for the details.
http://gtuinfo.in/Engineering-Degree/ExamPapers/ExamPapers.aspx

In this page, we need to select Branch, Semester to get exam papers.

In my CURL method, I am able to pass semester only. I am passing all fields as per VIEWSTATE method used in that page but it doesn't work anyhow.

My code:
<?php

$url = 'http://www.gtuinfo.in/Engineering-Degree/ExamPapers/ExamPapers.aspx';

$fields = array(
'ctl00$ContentPlaceHolder1$ddlBranch' => '7',
'ctl00$ContentPlaceHolder1$ddlSemester' => '6',
'ctl00$ContentPlaceHolder1$ddlExam' => '160511',
'__EVENTTARGET' => '',
'__EVENTARGUMENT' => '',
'__VIEWSTATE' => '',
'ctl00$ContentPlaceHolder1$btnSubjectResult' => 'Show',
'ctl00$ContentPlaceHolder1$btnBranchSemesterExam' => 'Show',
'__SCROLLPOSITIONX' => '0',
'__SCROLLPOSITIONY' => '0',
'ctl00$ContentPlaceHolder1$ddlSubject' => '-99',
);

$fields_string = "";
foreach($fields as $key=>$value)
{
$fields_string .= $key.'='.$value.'&';
}
rtrim($fields_string, '&');
$ch = curl_init();
curl_setopt($ch,CURLOPT_URL, $url);
curl_setopt($ch,CURLOPT_POST, count($fields));
curl_setopt($ch,CURLOPT_POSTFIELDS, $fields_string);

$result = curl_exec($ch);

curl_close($ch);

?>

Please provide the solution.