For lithuanian text with utf-8 encoding I use two functions (thanks [mattalexxpub at gmail dot com] and Svetoslav Marinov)

<?php
function my_ucfirst($string, $e ='utf-8') {
    if (function_exists('mb_strtoupper') && function_exists('mb_substr') && !empty($string)) {
        $string = mb_strtolower($string, $e);
        $upper = mb_strtoupper($string, $e);
            preg_match('#(.)#us', $upper, $matches);
            $string = $matches[1] . mb_substr($string, 1, mb_strlen($string, $e), $e);
    }
    else {
        $string = ucfirst($string);
    }
    return $string;
}

function sentence_case($string) {
    $sentences = preg_split('/([.?!]+)/', $string, -1, PREG_SPLIT_NO_EMPTY|PREG_SPLIT_DELIM_CAPTURE);
    $new_string = '';
    foreach ($sentences as $key => $sentence) {
        $new_string .= ($key & 1) == 0?
            my_ucfirst(strtolower(trim($sentence))) :
            $sentence.' ';  
    }
    return trim($new_string);
}
?>