Remove UTF-8 bad bytes

gistfile1.php

<?php
$text = "あいうえお";
$text1 = substr($text, 1);
$text2 = substr($text, 2);

echo mb_strimwidth($text1, 0, 10) . "\n";
echo mb_strimwidth($text2, 0, 10) . "\n";

function valid($text) {
    return (ord($text[0]) & 0xa0) != 0x80;
}
function validate($text) {
    while (!valid($text)) {
        $text = substr($text, 1);
    }
    return $text;
}

echo mb_strimwidth(validate($text1), 0, 10) . "\n";
echo mb_strimwidth(validate($text2), 0, 10) . "\n";