Multipart file uploads in PHP from strings

MultiPartFromStrings.php

<?php

/**
 * PHP's curl extension won't let you pass in strings as multipart file upload bodies; you
 * have to direct it at an existing file (either with deprecated @ syntax or the CURLFile
 * type). You can use php://temp to get around this for one file, but if you want to upload
 * multiple files then you've got a bit more work.
 * 
 * This function manually constructs the multipart request body from strings and injects it
 * into the supplied curl handle, with no need to touch the file system.
 * 
 * @param $ch resource curl handle
 * @param $boundary string a unique string to use for the each multipart boundary
 * @param $fields string[] fields to be sent as fields rather than files, as key-value pairs
 * @param $files string[] fields to be sent as files, as key-value pairs
 * @return resource the curl handle with request body, and content type set
 * @see http://stackoverflow.com/a/3086055/2476827 was what I used as the basis for this
 **/
function buildMultiPartRequest($ch, $boundary, $fields, $files) {
    $delimiter = '-------------' . $boundary;
    $data = '';

    foreach ($fields as $name => $content) {
        $data .= "--" . $delimiter . "\r\n"
            . 'Content-Disposition: form-data; name="' . $name . "\"\r\n\r\n"
            . $content . "\r\n";
    }
    foreach ($files as $name => $content) {
        $data .= "--" . $delimiter . "\r\n"
            . 'Content-Disposition: form-data; name="' . $name . '"; filename="' . $name . '"' . "\r\n\r\n"
            . $content . "\r\n";
    }

    $data .= "--" . $delimiter . "--\r\n";

    curl_setopt_array($ch, [
        CURLOPT_POST => true,
        CURLOPT_HTTPHEADER => [
            'Content-Type: multipart/form-data; boundary=' . $delimiter,
            'Content-Length: ' . strlen($data)
        ],
        CURLOPT_POSTFIELDS => $data
    ]);

    return $ch;
}

// and here's how you'd use it

$ch = curl_init('http://httpbin.org/post');
$ch = buildMultiPartRequest($ch, uniqid(),
    ['key' => 'value', 'key2' => 'value2'], ['somefile' => 'contents!', 'someotherfile' => 'yoloswag']);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
echo curl_exec($ch);