Freebase API call works in browser but fails with curl
My guess is that you are seeing NULL because the curl_exec returns false and then the json_decode converts the false to NULL.
This means, as you suspected, that curl_exec is failing. I'm not sure why, but I would start by debugging with curl_error() and curl_errno() to get more information on what is causing curl to fail.
Reference: curl_exec() always returns false