For those who, like me, lost a lot of minutes (hours) to understand why fwrite doesn't create a real utf-8 file, here's the explanation I've found :

I tried to do something like this :

[PHP] pobierz, plaintext 
<?php 
$myString = utf8_encode("Test with accents éčŕç"); 
$fh=fopen('test.xml',"w"); 
fwrite($fh,$myString); 
fclose($fh); 
?>
[PHP] pobierz, plaintext 

For a mysterious reason, the resulted file shows the accent without the utf-8 conversion.

I tried the binary, mode, etc. etc. And finally I've found it :
It seems that fwrite NEEDS to have the utf8_encode function INSIDE its parameters like this, to understand it must create a non-text only file :

[PHP] pobierz, plaintext 
<?php 
$myString = "Test with accents éčŕç"; 
$fh=fopen('test.xml',"w"); 
fwrite($fh,utf8_encode($myString)); 
fclose($fh); 
?>
[PHP] pobierz, plaintext 

Hope this will help