cURL with PHP and JSON - php

Question

I wish to mimic, using CURL with PHP, the operation of a website that retrieves data using an AJAX POST.
Normally when I'm viewing POST requests using Firebug you will see variable/value pairs, but in this case all you see is a single JSON string. E.g.
{"refId":"14536"}
Is there a way to mimic this request using CURL? I've looked at CURL but as far as I can see the CURLOPT_POSTFIELDS parameter has to be a query string made up of one or more name/value.
Here is my test code with a normal POST request using a single name/value pair. I'd like to modify it to do the above.
$curlOptions = array(
CURLOPT_RETURNTRANSFER => true, // return web page
CURLOPT_HEADER => false, // don't return headers
CURLOPT_ENCODING => "", // handle all encodings
CURLOPT_USERAGENT => "Mozilla/5.0 (Windows; U; Windows NT 6.0; en-GB; rv:1.9.0.3) Gecko/2008092417 Firefox/3.0.3",
CURLOPT_CONNECTTIMEOUT => 600, // timeout on connect
CURLOPT_TIMEOUT => 600, // timeout on response
CURLOPT_POST => true,
CURLOPT_POSTFIELDS => 'var1=113',
CURLOPT_URL => "http://localhost/t4.php"
);
$curlCh = curl_init();
curl_setopt_array( $curlCh, $curlOptions );
$fileContents = curl_exec( $curlCh );
$curlErr = curl_errno( $curlCh );
$curlErrmsg = curl_error( $curlCh );
if( $curlErr ) echo "CURL ERROR:</b> $curlErr $curlErrmsg";
echo $fileContents; //check worked
curl_close( $curlCh );

Answer 1

How about something like:
$postData = json_encode(array('refId' => '14536'));
curl_setopt($ch, CURLOPT_POSTFIELDS, $postData);