Lots of tries and lots of reading http-headers...

If you want to post $_POST vars and (in this case) one file named userfile to $remote_server and $remote_url.

<?php
        // get the necessary data
        $file_name = $_FILES['userfile']['name'];     // the file
        $tmp_name = $_FILES['userfile']['tmp_name'];     // the file
        $content_type = $_FILES['userfile']['type'];     // the file mime type
        
        srand((double)microtime()*1000000);
        $boundary = "---------------------".substr(md5(rand(0,32000)),0,10);
        
        // Build the header
        $header = "POST $remote_url HTTP/1.0\r\n";
        $header .= "Host: $remote_server\r\n";
        $header .= "Content-type: multipart/form-data, boundary=$boundary\r\n";
        // attach post vars
        foreach($_POST AS $index => $value){
            $data .="--$boundary\r\n";
            $data .= "Content-Disposition: form-data; name=\"".$index."\"\r\n";
            $data .= "\r\n".$value."\r\n";
            $data .="--$boundary\r\n";
        }
        // and attach the file
        $data .= "--$boundary\r\n";
        $content_file = join("", file($tmp_name));
        $data .="Content-Disposition: form-data; name=\"userfile\"; filename=\"$file_name\"\r\n";
        $data .= "Content-Type: $content_type\r\n\r\n";
        $data .= "".$content_file."\r\n";
        $data .="--$boundary--\r\n";
        $header .= "Content-length: " . strlen($data) . "\r\n\r\n";
                 // Open the connection
        $fp = fsockopen($remote_server, 80);
        // then just 
        fputs($fp, $header.$data);
        fclose($fp);
?>