For a 404 response to actually trigger an error as the example seems to be trying to demonstrate the following option should be set:
curl_setopt($ch,CURLOPT_FAILONERROR,true);
As per http://curl.haxx.se/libcurl/c/libcurl-errors.html
CURLE_HTTP_RETURNED_ERROR (22)
This is returned if CURLOPT_FAILONERROR is set TRUE and the HTTP server returns an error code that is >= 400. (This error code was formerly known as CURLE_HTTP_NOT_FOUND.)