$HTTP_RAW_POST_DATA and WordPress media_handle_upload

So I’m making something for which users need to upload images from a canvas element (on the front end) using WordPress’s in-built Media Upload API.

I’ve successfully uploaded images with the API from a file input in a form:

<input type="file" name="async-upload" id="async-upload" size="40" />
...
<?php $new_attach_id = media_handle_upload( 'async-upload', $new_id ); ?>

and I’ve saved images from the canvas using my own script:

<script type="text/javascript">
...
var img = canvas.toDataURL("image/png");
...
ajax.send(img );
</script>
...
<?php 
$imageData=$GLOBALS['HTTP_RAW_POST_DATA'];
$filteredData=substr($imageData, strpos($imageData, ",")+1);
$unencodedData=base64_decode($filteredData);
$fp = fopen( 'saved_images/canv_save_test.png', 'wb' );
fwrite( $fp, $unencodedData);
...
?>

Problem is, WordPress’s media_handle_upload() only accepts an index to the $_FILES array for the upload.

So the question is: How can I pass the image data from HTTP_RAW_POST_DATA to it? Could I make the $_FILES[‘tmp-name’] point to it somehow? Could I use another script as an intermediate step somehow? Could I unleash the monkeys on the typewriter until they come up with the answer?

Any help very very much appreciated!

Thanks,
Kane

EDIT: Solved

Thanks to @alex for his file_put_contents suggestion, and answers to another related question I posted here I now have this working. Solution: Send the unaltered base64 image data in the HTTP request, overwrite the tmp_file PHP generates and turn some security checks off for WP. Don’t know how safe that is but here’s the code for anyone who has the same problem:

<script type="text/javascript">
$('#save').click(function() {

        // get image data from cavas
        var img = canvas.toDataURL("image/png");

        // open ajax request
        var ajax = new XMLHttpRequest();
        ajax.open("POST",script_url,false); 
        //ajax.open("POST","/dumpvars.php",false); // testing

        // set headers
        boundary_str = "AJAX-------------" + (new Date).getTime();
        ajax.setRequestHeader('Content-Type', "multipart/form-data; boundary=" + boundary_str);

        // callback for completed request
        ajax.onreadystatechange=function()
        {
            if (ajax.readyState == 4)
            { 
                // Write out the filename.
                $("#ajax_out").html("Response: "+ajax.responseText+" End Response");
            }
        }

        // BUILD REQUEST
        var boundary = '--' + boundary_str; 
        var request_body = boundary + 'n'
        // print all html form fields
        $('#save_form input').each(function(){ 
            request_body += 'Content-Disposition: form-data; name="' 
            + $(this).attr('name') + '"' + 'n' 
            + 'n' 
            + $(this).val() + 'n' 
            + 'n' 
            + boundary + 'n';
        });
        // make filename
        var filename = $('#save_form input[name="title"]').val();
        filename = filename.replace(/s+/g, '-').toLowerCase(); // hyphenate + lowercase
        filename = encodeURIComponent(filename) + (new Date).getTime() + ".png";
        // add image
        request_body += 'Content-Disposition: form-data; name="async-upload"; filename="' 
            + filename + '"' + 'n'
        + 'Content-Type: image/png' + 'n' 
        + 'n' 
        + img
        + 'n' 
        + boundary;

        // Send request
        ajax.send(request_body);

    });
</script>

...


<?php
    // Get transmitted image data 
    $loc = $_FILES['async-upload']['tmp_name'];
    $file = fopen($loc, 'rb');
    $contents = fread($file, filesize($loc));
    fclose($file);
    // Decode image data
    $filteredData=substr($contents, strpos($contents, ",")+1);
    $unencodedData=base64_decode($filteredData);
    // Overwrite temp file
    file_put_contents($loc, $unencodedData);

    // Pass image data to WP Upload (attach to post)
    $override['test_upload'] = false; // Override WP's upload security test (rewriting tmp_name made it fail)
    $override['test_form'] = false;
    $new_attach_id = media_handle_upload( 'async-upload', $new_id, array() , $override );

?>