@DougW has clearly answered this question, but I still like to add some codes here to explain Doug's points. (And correct errors in the code above)
Solution 1: URL-encode the POST data with a content-type header :application/x-www-form-urlencoded .
Note: you do not need to urlencode $_POST[] fields one by one, http_build_query() function can do the urlencoding job nicely.
$fields = array(
'mediaupload'=>$file_field,
'username'=>$_POST["username"],
'password'=>$_POST["password"],
'latitude'=>$_POST["latitude"],
'longitude'=>$_POST["longitude"],
'datetime'=>$_POST["datetime"],
'category'=>$_POST["category"],
'metacategory'=>$_POST["metacategory"],
'caption'=>$_POST["description"]
);
$fields_string = http_build_query($fields);
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL,$url);
curl_setopt($ch, CURLOPT_POST,1);
curl_setopt($ch, CURLOPT_POSTFIELDS,$fields_string);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$response = curl_exec($ch);
Solution 2: Pass the array directly as the post data without URL-encoding, while the Content-Type header will be set to multipart/form-data.
$fields = array(
'mediaupload'=>$file_field,
'username'=>$_POST["username"],
'password'=>$_POST["password"],
'latitude'=>$_POST["latitude"],
'longitude'=>$_POST["longitude"],
'datetime'=>$_POST["datetime"],
'category'=>$_POST["category"],
'metacategory'=>$_POST["metacategory"],
'caption'=>$_POST["description"]
);
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL,$url);
curl_setopt($ch, CURLOPT_POST,1);
curl_setopt($ch, CURLOPT_POSTFIELDS,$fields);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$response = curl_exec($ch);
Both code snippets work, but using different HTTP headers and bodies.