If you know the data is in UTF8 than you want to set up the header.

I wrote a solution answering to another tread.

The solution is the following: As the UTF-8 byte-order mark is \xef\xbb\xbf we should add it to the document's header.

<?php

function writeStringToFile($file, $string){

    $f=fopen($file,"wb");

    $file="\xEF\xBB\xBF".$file;// this is what makes the magic

    fputs($f, $string);

    fclose($f);}?>

You can adapt it to your code, basically you just want to make sure that you write a UTF8 file (as you said you know your content is UTF8 encoded).