Country code to country flag Emoji (PL -> 🇵🇱)

flag.php

<?php

/**
 * Converts country code (ISO 3166-1) to its emoji flag representation (PL -> 🇵🇱).
 *
 * This solution supports both lowercase and uppercase codes using modulo 32 .
 * Since it doesn't perform any validation, you should make sure the code is a valid country code first.
 *
 * 0x1F1E5 is a code of character right before "REGIONAL INDICATOR SYMBOL LETTER A" (🇦).
 *
 * Since there are 32 letters in the ASCII and A is at code 65 (dec), modulo operation returns:
 *  - 1 for "A" and "a"
 *  - 2 for "B" and "b"
 *  etc.
 */

function country2flag(string $countryCode): string
{
    return (string) preg_replace_callback(
        '/./',
        static fn (array $letter) => mb_chr(ord($letter[0]) % 32 + 0x1F1E5),
        $countryCode
    );
}

echo country2flag('PL'); // 🇵🇱
echo country2flag('SG'); // 🇸🇬
echo country2flag('us'); // 🇺🇸