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));