Captcha in Cyrillic

I make captcha in Cyrillic from ttf fonts. php development language I set an array of Latin and (or) numbers - it works. I set an array of Cyrillic - krakozyabry. What could be the error?

function vam_random_charcode($length) { $arraysize = 33; $chars = array('1','2','3','4','5','6','7','8','9','А','Б','В','Г','Д','Е','Ж','И','Й','К','Л','М','Н','П','Р','С','Т','У','Ф','Х','Ц','Ч','Ы','Э','Ю','Я'); $code = ''; for ($i = 1; $i <= $length; $i++) { $j = floor(vam_rand(0,$arraysize)); $code .= $chars[$j]; } return $code; }