utf8_charcodes.php

<?php

function utf8_to_charcode(string $utf8, &$i = null): int {
    $i = (int) $i;
    if ($i >= strlen($utf8)) {
        return -1;
    }
    $first = ord($utf8[$i]);
    if ($first >> 3 === 0xf << 1) {
        $code = $first & 0x7;
        for ($j = 0; $j < 3; $j++) { 
            $i++;
            if ($i >= strlen($utf8)) {
                return -1;
            }
            $byte = ord($utf8[$i]);
            if ($byte >> 6 !== 0x2) {
                return -1;
            }
            $code = $code << 6 | $byte & 0x3f;
        }
        return $code;
    }
    if ($first >> 4 === 0x7 << 1) {
        $code = $first & 0xf;
        for ($j = 0; $j < 2; $j++) {
            $i++;
            if ($i >= strlen($utf8)) {
                return -1;
            }
            $byte = ord($utf8[$i]);
            if ($byte >> 6 !== 0x2) {
                return -1;
            }
            $code = $code << 6 | $byte & 0x3f;
        }
        return $code;
    }
    if ($first >> 5 === 0x3 << 1) {
        $i++;
        if ($i >= strlen($utf8)) {
            return -1;
        }
        $second = ord($utf8[$i]);
        if ($second >> 6 !== 0x2) {
            return -1;
        }
        return $first & 0x1f << 6 | $second & 0x3f;
    }
    if ($first >> 7 === 0x0) {
        return $first;
    }
    return -1;
}


function utf8_from_charcode(int $code): string {
    if ($code > 0x1fffff || $code < 0) {
        return '';
    }
    if ($code > 0xffff) {
        $str = '';
        for ($i = 0; $i < 3; $i++) {
            $str = chr(0x80 | $code & 0x3f) . $str;
            $code >>= 6;
        }
        return chr(0xf0 | $code) . $str;
    }
    if ($code > 0x7ff) {
        $str = '';
        for ($i = 0; $i < 2; $i++) {
            $str = chr(0x80 | $code & 0x3f) . $str;
            $code >>= 6;
        }
        return chr(0xe0 | $code) . $str;
    }
    if ($code > 0x7f) {
        $str = chr(0x80 | $code & 0x3f);
        $code >>= 6;
        return chr(0xc0 | $code) . $str;
    }
    return chr($code);
}