How do I write UTF-8 data to a UTF-16LE file using PHP?

Given a string of UTF-8 data in PHP, how can I convert and save it to a UTF-16LE file (this particular file happens to be destined for Indesign - to be placed as a tagged text document).

Data:

$copy = "<UNICODE-MAC>\n";
$copy .= "<Version:8><FeatureSet:InDesign-Roman><ColorTable:=<Black:COLOR:CMYK:Process:0,0,0,1>>\n";
$copy .= "A bunch of unicode special characters like ñ, é, etc.";

I am using the following code, but to no avail:

file_put_contents("output.txt", pack("S",0xfeff) . $copy);

Using the following code, I have found a solution:

this function changes the byte order (from http://shiplu.mokadd.im/95/convert-little-endian-to-big-endian-in-php-or-vice-versa/):

function chbo($num) {
    $data = dechex($num);
    if (strlen($data) <= 2) {
        return $num;
    }
    $u = unpack("H*", strrev(pack("H*", $data)));
    $f = hexdec($u[1]);
    return $f;
}

used with a utf-8 to utf-16LE conversion, it creates a file that will work with indesign:

file_put_contents("output.txt", pack("S",0xfeff). chbo(iconv("UTF-8","UTF-16LE",$copy));