How to make Stripos("UTF-8") in PHP?

Question

There is situation like this:

<?php
    $string="Tu gdzieś jest ten ŁAŻĄCY po lesie troll";
    //translatePL2EN($string)=="Somewhere here is the troll, who USUALLY WALKS through the forest."
    $result = stripos($string,'łażący');
    $result2 = mb_stripos($string,'łażący');
?>

I wanted to find case insensitive version of ŁAŻĄCY - łażący using stripos(); or mb_stripos();

it should return int(20), but it returns false.

How to fix that?

stripos returns false when special characters is used this still not fix the problem.

Even this

setlocale(LC_ALL, 'pl_PL', 'pl', 'Polish_Poland.28592');
$result = mb_stripos("ĘÓĄŚŁŻŹĆŃ",'ęóąśłżźćń');

returns false...