- mb_substr: documentation ( source)
- mb_strlen: documentation ( source)
- mb_strtoupper: documentation ( source)
<?php
$str = "моя строка";
$encoding = "UTF-8";
$first_letter = mb_strtoupper(mb_substr($str, 0, 1, $encoding), $encoding);
$str = $first_letter . mb_substr($str, 1, mb_strlen($str, $encoding), $encoding);
echo $str;