test.php

<?php 
echo '<meta http-equiv="Content-Type" content="text/html; charset=utf-8">';

function get_fname($string, $sub) {
	$bad = array("<", ">", ":", '"', "/", "\\", "|", "?", "*");
	$name = str_replace($bad, "_", $string);
	echo strlen(utf8_decode($string));
	if (strlen($string) > $sub) {
		$name = mb_substr($string,0,$sub, "utf-8");
	}
	return $name;
}

$test_string = "gëén";
$sub_arr = array(1,2,3);

foreach ($sub_arr as $sub) {
	echo get_fname($test_string, $sub);
	echo "<br />";
}

?>