mb_substr - 如何让它工作？

I'm creating a conjugator. When the user inputs a verb, the program changes the last 2 characters with corresponding endings for each person.

The problem is that it only works without special characters - if I use any special characters, it doesn't return anything. This is the code:

<?php
$verb = $_POST["verb"];
$last2 = substr($verb, -2);
$aic_eic_oic = array("je", "isz", "i", "imy", "icie", "ja" );  

if ($last3 == "aić" || $last3 == "eić" || $last3 == "oić") {
    foreach ($aic_eic_oic as $one) {
        echo "<li>$root2$one</li>";
        }   
    }

?>

I know already that substr only works with one character = one byte which is not the case with special characters, so I've been experimenting with mb_substr, but without success. The important thing is that the lenght of the verb will vary.

I'm sorry for duplicating my own question from a few months ago - I got a partial solution then, but unfortunately I cannot access it from this computer.