Besides uploading file with `@{file_name}` as `CURLOPT_POSTFIELDS` in curl, uploading multiple files with file content string is feasible. The way is to build the POST headers manually.

curl_upload_files.php

<?php
// form field separator
$delimiter = '-------------' . uniqid();
// file upload fields: name => array(type=>'mime/type',content=>'raw data')
$fileFields = array(
	// For multiple files, name must be suffixed with '[]'.
    'file[]' => array(
        array(
			'name' => 'file1.txt',
			'type' => 'text/plain',
			'content' => '...your raw file content goes here...'
		),
		array(
			'name' => 'file2.txt',
			'type' => 'text/plain',
			'content' => '...your raw file content goes here...'
		),
	),
	'file2' => array(
		'name' => 'file2.txt',
		'type' => 'text/plain',
		'content' => '...your raw file content goes here...'
	)
);
// all other fields (not file upload): name => value
$postFields = array(
    'otherformfield' => 'content of otherformfield is this text',
    /* ... */
);

$data = '';

// populate normal fields first (simpler)
foreach ($postFields as $name => $content) {
   $data .= "--" . $delimiter . "\r\n";
    $data .= 'Content-Disposition: form-data; name="' . $name . '"';
    // note: double endline
    $data .= "\r\n\r\n";
	$data .= $content . "\r\n";
}
// populate file fields
foreach ($fileFields as $name => $file) {
	if (isset($file['content']))
	{
		$file = array($file);
	}

	foreach ($file as $f)
	{
		 $data .= "--" . $delimiter . "\r\n";
		// "filename" attribute is not essential; server-side scripts may use it
		$data .= 'Content-Disposition: form-data; name="' . $name . '";' .
				 ' filename="' . $f['name'] . '"' . "\r\n";
		// this is, again, informative only; good practice to include though
		$data .= 'Content-Type: ' . $f['type'] . "\r\n";
		// this endline must be here to indicate end of headers
		$data .= "\r\n";
		// the file itself (note: there's no encoding of any kind)
		$data .= $f['content'] . "\r\n";
	}
}
// last delimiter
$data .= "--" . $delimiter . "--\r\n";

$url = 'http://localhost/emails/server.php';
$handle = curl_init($url);
curl_setopt($handle, CURLOPT_POST, true);
curl_setopt($handle, CURLOPT_HTTPHEADER , array(
    'Content-Type: multipart/form-data; boundary=' . $delimiter,
    'Content-Length: ' . strlen($data)));  
curl_setopt($handle, CURLOPT_POSTFIELDS, $data);
$response = curl_exec($handle);

Referred http://stackoverflow.com/questions/3085990/post-a-file-string-using-curl-in-php