If curl doesn't return anything then you have to check for the reason why that happen. Check for the last curl error message:

if(curl_exec($ch) === false)
{
    echo 'Curl error: ' . curl_error($ch);
}
else
{
    echo 'Operation completed without any errors, you have the response';
}

Basically it would be good to always check on that. You shouldn't assume that curl request succeeded.

Two of your variables are named badly:

curl_setopt($ch, CURLOPT_USERAGENT, config['userAgent']);
curl_setopt($ch, CURLOPT_REFERER, config['siteURL']);

Should be:

curl_setopt($ch, CURLOPT_USERAGENT, $config['userAgent']);
curl_setopt($ch, CURLOPT_REFERER, $config['siteURL']);

My guess is that PHP faulted to a blank page (since the variable is seen as a constant but constants must be scalar).

Another issue may be the user agent. I've seen servers that completely refuse to reply if no user agent was set.

You can use a class that I wrote that wraps around CURL. To install, see:

It's much easier to use and can give you some easy to access debugging. For example:

try{

    //load class autoloader
        require_once( __DIR__ . '/loader.php' );  //you should ensure that is is the correct path for this file

    //make the response; return the response with the headers
        $client = new \Altumo\Http\OutgoingHttpRequest( ' array(
            'username' => 'user',
            'password' => 'pass',
            'submit' => ' Login '
        ));
        $client->setRequestMethod( \Altumo\Http\OutgoingHttpRequest::HTTP_METHOD_POST );

    //send the request (with optional arguments for debugging)           
        //the first true will return the response headers
        //the second true will turn on curl info so that it can be retrieved later
        $response = $client->send( true, true );

    //output the response and curl info
        \Altumo\Utils\Debug::dump( $response, $client->getCurlInfo() );

    //alternatively, you can get the response wrapped in an object that allows you to retrieve parts of the response
        $http_response =  $client->sendAndGetResponseMessage( true );
        $status_code = $http_response->getStatusCode();
        $message_body = $http_response->getMessageBody();
        $full_http_response = $http_response->getRawHttpResponse();
        \Altumo\Utils\Debug::dump( $status_code, $message_body, $full_http_response );


}catch( \Exception $e ){

    //This will display an error if any exceptions have been thrown
    echo 'Error: ' . $e->getMessage();

}

Hope that helps...