is_utf8.php

<?php

/**
 * @param string $str
 * @param bool $truncated
 * @return bool
 */
function is_utf8(string $str, bool $truncated = false): bool
{
        $length = strlen($str);
        if ($length == 0) {
            return true;
        }
        for ($i = 0; $i < $length; $i++) {
            $high = ord($str{$i});
            if ($high < 0x80) {
                continue;
            } elseif ($high <= 0xC1) {
                return false;
            } elseif ($high < 0xE0) {
                if (++$i >= $length) {
                    return $truncated;
                } elseif (($str{$i} & "\xC0") == "\x80") {
                    continue;
                }
            } elseif ($high < 0xF0) {
                if (++$i >= $length) {
                    return $truncated;
                } elseif (($str{$i} & "\xC0") == "\x80") {
                    if (++$i >= $length) {
                        return $truncated;
                    } elseif (($str{$i} & "\xC0") == "\x80") {
                        continue;
                    }
                }
            } elseif ($high < 0xF5) {
                if (++$i >= $length) {
                    return $truncated;
                } elseif (($str{$i} & "\xC0") == "\x80") {
                    if (++$i >= $length) {
                        return $truncated;
                    } elseif (($str{$i} & "\xC0") == "\x80") {
                        if (++$i >= $length) {
                            return $truncated;
                        } elseif (($str{$i} & "\xC0") == "\x80") {
                            continue;
                        }
                    }
                
                }
            } // F5~FF is invalid by RFC 3629
            
            return false;
        }

        return true;
    }