How to POST JSON data via PHP?

I've used curl before to post "field" data to a server, but in this case, I actaully need to post JSON data. However, when I run this PHP script, I get nothing back. If I do this curl manually in Terminal, it works. I'm pretty sure the bug has something to do with CURLOPT_POSTFIELDS - - - I just don't know what the alternative is. Basically I need to post this data: {siteUrl:\"http://testurl.com\",siteRss:\"http://feeds.news.mysite.com/synfeeds/collect/6/rss.xml\",userName:\"testuser\",email:\"testemail\",siteName:\"testsitename\",loginId:\"testloginid\} to the url: http://dpdev-ld03.myservercom:8181/linkmethod/signup?f=json&c=signupcallback Any obvious mistakes below? <?php $ch = curl_init(); $timeout = 10; // set to zero for no timeout $urltouse = "http://dpdev-ld03.myservercom:8181/linkmethod/signup?f=json&c=signupcallback"; $fields_string = '{siteUrl:\"http://testurl.com\",siteRss:\"http://feeds.news.mysite.com/synfeeds/collect/6/rss.xml\",userName:\"testuser\",email:\"testemail\",siteName:\"testsitename\",loginId:\"testloginid\}'; curl_setopt($ch,CURLOPT_URL,$urltouse); curl_setopt($ch,CURLOPT_POST,count($fields)); curl_setopt($ch,CURLOPT_POSTFIELDS,$fields_string); <-- shouldn't be postfields, but some other curl option I'm guessing $response = curl_exec($ch); curl_close($ch); echo $response; ?>