cURL File upload array

I'm trying to upload files via cURL and it works fine except when the file field is an array - in that case it just sends 'array' and it ends up as a regular post field.

$ch = curl_init('http://example.com/files_test.php');

curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_0);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION ,1);

if(count($_POST)>0 || count($_FILES)>0)
{
foreach($_POST as $key=>$val)
{
$params[$key]=$val;
}
foreach($_FILES as $key=>$val)
{
if(is_array($val['tmp_name']))
{
foreach($val['tmp_name'] as $id=>$filename)
{
if($val['error'][$id]==0)
{
$params[$key][$id]='@'.$filename;
}
}
}
else
{
$params[$key]='@'.$val['tmp_name'];
}
}
curl_setopt($ch, CURLOPT_POST,1);
curl_setopt($ch, CURLOPT_POSTFIELDS,$params);
}

$data = curl_exec($ch);
$content_type = curl_getinfo( $ch, CURLINFO_CONTENT_TYPE );
curl_close($ch);


before the curl call the content of $params is correct - eg.

Array
(
[upload_file1] => @/tmp/phpQVxDa0
[upload_file2] => @/tmp/php5KiAPY
[upload_file3] => Array
(
[0] => @/tmp/phppf1Z24
[1] => @/tmp/phptbYhhl
)

)


but after the curl call $parms has been altered to:

Array
(
[upload_file1] => @/tmp/phpQVxDa0
[upload_file2] => @/tmp/php5KiAPY
[upload_file3] => Array
)

Is there any way around this? Also - will the same happen to regular post fields?