The website you are trying to request from throws an error when using cURL because the google tools they have implemented in their python code crash when there is no user agent set.
Try adding this line to your code before
curl_setopt($ch,CURLOPT_USERAGENT,'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:18.104.22.168) Gecko/20080311 Firefox/22.214.171.124');
And as @jeroen said, using
json_decode($content, true) is not neccesary as the returned data will be HTML code not a json string. Remove that line as well and you should be good to go.