使用PHP通过cURL发布XML而无需先生成XML文件

I am trying to send an XML file via POST to a form on a remote server. I am generating the XML data on the fly, and storing it as a string in a variable called $XmlData. This is a string starting with my opening tag (see example below)...so no "headers" are in this string. I could easily write it to the server in a temp directory as an XML file...but ideally i want to just post this as if it were a file, but without the added step of creating the file (which i would then have to delete afterwords)... Below is the code that i THINK should allow me to upload my XML file if i actually created it first...

    $url  = 'http://www.mydomain.com/path/to/form/process.php';

    $ch = curl_init($url);
    curl_setopt($ch, CURLOPT_HEADER, 0);
    curl_setopt($ch, CURLOPT_VERBOSE, 0);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_USERAGENT, "Mozilla/4.0 (compatible;)");
    curl_setopt($ch, CURLOPT_URL, _VIRUS_SCAN_URL);
    curl_setopt($ch, CURLOPT_POST, true);
    // same as <input type="file" name="xmlfileresource">
    $post = array(
        "xmlfileresource"=>"@/path/to/myfile.xml",
        "action"=>"incomingXml",
        "user"=>"JohnSmith",
    );
    curl_setopt($ch, CURLOPT_POSTFIELDS, $post); 
    $response = curl_exec($ch);
    curl_close($ch);

My question is this: How can I stream/blob/whatever the content of my $XmlData string into this cURL execution such that the remote server thinks it was just a normal file being POSTed?

Not really relevant, but in case it helps...my $XmlData string might look something like this:

<clientversion>
    <component>
    <type>module</type>
    <name>resources</name>
    <version>2.7.3</version>
    <date>2012-09-01 02:18:33</date>
    </component>
    <component>
    <type>module</type>
    <name>staff</name>
    <version>3.1</version>
    <date>2011-04-01 07:12:48</date>
    </component>
</clientversion>