A toy for https://www.v2ex.com/t/510728

IntEncoder.php

<?php

declare(strict_types=1);

class IntEncoder
{
    /**
     * @var string all unreserved chars in URI
     *
     * @see https://en.wikipedia.org/wiki/Percent-encoding#Percent-encoding_in_a_URI
     */
    const CHARS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_.~';

    /**
     * Encode the string.
     *
     * @param string $raw the raw integer string
     *
     * @return string
     */
    public static function encode(string $raw): string
    {
        $base = (string) strlen(static::CHARS);

        $encoded = '';

        do {
            $remainder = bcmod($raw, $base);
            $encoded .= static::CHARS[$remainder];
            $raw = bcdiv($raw, $base); // quotient
        } while (bccomp($raw, '0') === 1);

        return $encoded;
    }

    /**
     * Decode the string.
     *
     * @param string $enc the encoded string
     *
     * @return string
     */
    public static function decode(string $enc): string
    {
        static $weightingMap; // cache

        $base = (string) strlen(static::CHARS);

        // build the weighting map for the first-time use
        $weightingMap = $weightingMap ?? array_combine(
            str_split(static::CHARS), // keys
            array_map('strval', range(0, (int) $base - 1)) // values
        );

        $decoded = '';

        for ($i = 0; $i < strlen($enc); ++$i) {
            $decoded = bcadd(
                $decoded,
                bcmul($weightingMap[$enc[$i]], bcpow($base, "{$i}"))
            );
        }

        return $decoded;
    }
}

$intString = '11918780727494821840';

$encoded = IntEncoder::encode($intString);
$decoded = IntEncoder::decode($encoded);

assert($decoded === $intString);

echo 'Encoded: ', $encoded, PHP_EOL;
echo '# of chars saved: ', strlen($intString) - strlen($encoded), PHP_EOL;

Output:

Encoded: W7tuQpX~lnH
# of chars saved: 9