Description:
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Using curl_setopt, you can make a multipart POST by passing an array of parameters, and if preceded by an @, a file upload is created from the path following the @.
  curl_setopt(CURLOPT_POSTFIELDS, array('myfile'=>"@{$pathname}"));
However, I haven't been able to find a way to set the filename part of the multipart description, so that it generates e.g.
  Content-disposition: form-data; name="myfile"; filename="desiredfilename"
  Content-Type: text/plain
That is, there isn't a way to get "desiredfilename" in there. The only option seems to be for the filename part appears to come from the name of the file within the path. The cURL command line interface seems to support explicitly setting the filename different from the name of the source file with 
'-F "@path ; filename=whatever'
so presumably it is possible in the library, but not exposed through the PHP binding.

In case it was merely passing on the string to the cURL library verbatim, I tried:
 curl_setopt(CURLOPT_POSTFIELDS, array('myfile'=>"@{$pathname}; filename=\"desiredfilename\""));
but that gives an error.

The problem is that the files I have are stored with simple numeric names and the file names which an external user would know them by are in the database that indexes the disk files. I have a workround which is to make a temporary hard link to the file by its user name and give that to cURL, but I don't think I should really need to do that.

Test script:
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    $now = time(); $tmpdirpath = "/tmp/enspub-{$now}-{$version->id}";
    mkdir($tmpdirpath);
    $tmpfilepath = "{$tmpdirpath}/" . mb_ereg_replace('[/\\\\?%*:|"<>@~=+]', '_', $version->name); // replace disallowed characters
    link($version->pathname(), $tmpfilepath);
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE); /* tell me, not echo */
    curl_setopt($ch, CURLOPT_POST, TRUE);
    $post['file'] = "@{$tmpfilepath}"; 
    /* ... and other $post settings ... */
    curl_setopt($ch, CURLOPT_POSTFIELDS, $post); 
    $data = curl_exec($ch);
    /* ... error checking ... */
    curl_close($ch);
    @unlink($tmpfilepath);
    @rmdir($tmpdirpath);



Expected result:
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It works, but I'd like to be able to set the filename explicitly without copying the file.

Actual result:
--------------
n/a