Calculate regional discount using Parity Purchasing Power for user IP address

discount.php

<?php

// Copyright (C) Jakub T. Jankiewicz
// this code is released to Public Domain with CC0 license
// created with the help from ChatGPT

$default_country = [
  'name' => 'Poland',
  'code' => 'PL'
];

function curl($url) {
    $ch = curl_init($url);
    curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
    curl_setopt($ch, CURLOPT_HEADER, 0);
    if (isset($_SERVER['HTTP_USER_AGENT'])) {
        $agent = $_SERVER['HTTP_USER_AGENT'];
    } else {
        // defaut FireFox 15 from agent switcher (google chrome extension)
        $agent = 'Mozilla/5.0 (Windows NT 6.1; WOW64; rv:15.0) Gecko/20120427 '.
                 'Firefox/15.0a1';
    }
    curl_setopt($ch, CURLOPT_USERAGENT, $agent);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    return $ch;
}


function remote_ip() {
    if (isset($_SERVER['HTTP_X_FORWARDED_FOR'])) {
        return $_SERVER['HTTP_X_FORWARDED_FOR'];
    }
    return $_SERVER['REMOTE_ADDR'];
}

function is_localhost($ip) {
    if ($ip === '127.0.0.1' || $ip === '::1') {
        return true;
    }
    if (substr($ip, 0, 7) === '::ffff:') {
        $ip = substr($ip, 7);
        if ($ip === '127.0.0.1') {
            return true;
        }
    }
    return false;
}

function get($url) {
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_USERAGENT, "PHP " . phpversion());
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    $data = curl_exec($ch);
    curl_close($ch);
    return $data;
}



// get country information
function get_country() {
    $ip = remote_ip();
    if (is_localhost($ip)) {
        return $default_country;
    }
    $url = "http://ip-api.com/json/$ip";
    $data = get($url);
    $data = json_decode(get($url));
    return [
        'name' => $data->country,
        'code' => $data->countryCode
    ];
}


// get Parity Purchasing Power value
function get_ppp($country) {
    $url = "https://api.worldbank.org/v2/country/$country/indicator/PA.NUS.PPP";
    $data = get($url);
    $xml = new SimpleXMLElement($data);

    foreach ($xml->xpath('//wb:value') as $value) {
        if (!empty($value)) {
            return floatval($value);
        }
    }
    return 1;
}

// calculate Parity Purchasing Power discount based on a country
function get_discount($price, $country) {
    $ppp = get_ppp($country);
    if ($ppp == 1) {
        return [
            'price' => $price
        ];
    }
    $new_price = $price / $ppp;
    $discount = ($price - $new_price) / $price * 100;
    return [
        'price' => '$' . round($new_price),
        'percent' => strval(round($discount)) . '%'
    ];
}

$country = get_country();

$price = 100;

$discount = get_discount($price, $country['code']);

$price_message = "The price is ${$price}";

if (isset($discount['percent'])) {
    $price_message .= " (but it looks like you are from <strong>{$country['name']}</strong>, ".
                        " so you can get {$discount['percent']} discount. And can pay {$discount['price']})";
}

echo "<p>$price_message</p>";

Here is a different approach with hardcoded values

NOTE: above code don't use any limits like minimal or maximal values. If the value is too small or too big you can add min() and/or max() functions.