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📄 Source: net.php

<?php
/**
 * Tiny network helpers: LAN detection, client IP.
 */

function client_ip(): string {
    // PHP built-in server sets REMOTE_ADDR reliably. Behind a proxy,
    // you'd also check X-Forwarded-For, but we deliberately don't trust it
    // by default (spoofable).
    return $_SERVER['REMOTE_ADDR'] ?? '0.0.0.0';
}

function ip_in_cidr(string $ip, string $cidr): bool {
    if (strpos($cidr, '/') === false) {
        return $ip === $cidr;
    }
    [$subnet, $bits] = explode('/', $cidr, 2);
    $bits = (int)$bits;

    $ip_long     = ip2long($ip);
    $subnet_long = ip2long($subnet);
    if ($ip_long === false || $subnet_long === false) {
        return false;
    }

    if ($bits < 0 || $bits > 32) {
        return false;
    }

    $mask = $bits === 0 ? 0 : (~0 << (32 - $bits));
    return ($ip_long & $mask) === ($subnet_long & $mask);
}

function is_lan(array $subnets): bool {
    $ip = client_ip();
    foreach ($subnets as $cidr) {
        if (ip_in_cidr($ip, $cidr)) {
            return true;
        }
    }
    return false;
}