You could use cURL or sockets to re-post the data, but you can't really redirect it.
POST'ing to a URL with cURL:
$ch = curl_init('http://www.somewhere.com/that/receives/postdata.php');
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($_POST));
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$response = curl_exec($ch);
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