<form action="#" method="POST" enctype="multipart/form-data" >
<input type="file" name="file[]">
<br>
<input type="file" name="file[]">
<br>
<input type="file" name="file[]">
<br>
<input type="submit" name="upload" value="Upload">
<br>
</form>

<?php
function fixFilesArray(&$files) {
    // a mapping of $_FILES indices for validity checking
    $names = array('name' => 1, 'type' => 1, 'tmp_name' => 1, 'error' => 1, 'size' => 1);
    // iterate over each uploaded file
    foreach ($files as $key => $part) {
    // only deal with valid keys and multiple files
        $key = (string) $key;
        if (isset($names[$key]) && is_array($part)) {
            foreach ($part as $position => $value) {
                $files[$position][$key] = $value;
            }
            // remove old key reference
            unset($files[$key]);
        }
    }
}

if (isset($_POST)) {

    fixFilesArray($_FILES['file']);

    // email fields: to, from, subject, and so on
    $to = "email@email.com";
    $from = "email@email.com";
    $subject ="My subject";
    $message = "My message";
    $headers = "From: $from";

    // boundary
    $semi_rand = md5(time());
    $mime_boundary = "==Multipart_Boundary_x{$semi_rand}x";

    // headers for attachment
    $headers .= "\nMIME-Version: 1.0\n" . "Content-Type: multipart/mixed;\n" . " boundary=\"{$mime_boundary}\"";

    // multipart boundary
    $message = "This is a multi-part message in MIME format.\n\n" . "--{$mime_boundary}\n" . "Content-Type: text/plain; charset=\"iso-8859-1\"\n" . "Content-Transfer-Encoding: 7bit\n\n" . $message . "\n\n";
    $message .= "--{$mime_boundary}\n";

    foreach ($_FILES['file'] as $file) {
        if (!EMPTY($file['name'])) {
            $fsize = $file['size'];
            $fname = $file['name'];
            $tmp_file = fopen($file['tmp_name'],"rb");
            $data = fread($tmp_file,$fsize);
            fclose($tmp_file);
            $data = chunk_split(base64_encode($data));
            $message .= "Content-Type: {\"application/octet-stream\"};\n" . " name=\"$fname\"\n" .
            "Content-Disposition: attachment;\n" . " filename=\"$fname\"\n" .
            "Content-Transfer-Encoding: base64\n\n" . $data . "\n\n";
            $message .= "--{$mime_boundary}\n";
        }
    }

    // send

    $ok = @mail($to, $subject, $message, $headers);
    if ($ok) {
    echo "<p>mail sent to $to!</p>";
    } else {
    echo "<p>mail could not be sent!</p>";
    }
}

?>