删除utf8字符的代码在php 5.3.3上不起作用

function remove_utf8_chars($string) {
    $result = '';
    for ($i = 0; $i < mb_strlen($string, 'UTF-8'); $i++) {
        $char = mb_substr($string, $i, 1, 'UTF-8');
        if (mb_check_encoding($char, 'UTF-8')) {
            continue;
        }
        $result .= $char;
    }
    return $result;
}

function remove_utf8_chars($string) {
    return preg_replace('/[\x00-\x7F\xC0-\xFD]/', '', $string);
}

function remove_utf8_chars($string) {
    return iconv('UTF-8', '', $string);
}