emoji-flag.php

<?php
// LICENSE: Public Domain

/**
 * Converts a letter to its corresponding regional indicator Unicode character
 * @param $chr string The letter to convert (A to Z, case insensitive). If outside this range, it silently wraps around in an undefined manner
 * @return string The regional indicator Unicode character
 */
function letter_to_regional_indicator($chr) {
    // 0x41 = capital letter A, while the modulo 26 clamps input to A-Z by wrapping around
    $distance_from_A = (ord(strtoupper($chr)) - 0x41) % 26;
    // 0x1F1E6 = regional indicator A
    return mb_chr(0x1F1E6 + $distance_from_A, 'UTF-8');
}

/**
 * Converts an ISO 3166-1 alpha-2 country code into its corresponding flag emoji
 * @param $code string The country code. If input is incorrect, it silently produces an incorrect flag
 * @return string The flag emoji characters. Unsupported combinations are rendered as the individual letters
 */
function country_code_to_flag_emoji($code) {
    return letter_to_regional_indicator($code[0]) . letter_to_regional_indicator($code[1]);
}