Simple php built-in webserver router script.

router.php

<?php

define('PUBLIC_DIR', __DIR__ . DIRECTORY_SEPARATOR . 'public');

$uri = $_SERVER['REQUEST_URI'];

// assume path exists in public directory
$path = PUBLIC_DIR . $uri;

// convert to realpath and substitute '.' and '..'
$real = realpath($path);
if ($real === false) {
    http_response_code(404);
    echo "resource not found";

    return true;
}

// check if we are still in the public dir ('..' could have been used to change directories)
if (mb_strpos($real, PUBLIC_DIR) === false) {
    http_response_code(400);
    echo "invalid uri";

    return true;
}

$exists = false;
if (is_dir($real)) {
    // check if the directory contains an index file
    foreach (['index.php', 'index.html', 'default.php', 'default.html'] as $indexFile)
        if (file_exists($real . DIRECTORY_SEPARATOR . $indexFile)) {
            // append index file path
            $real .= DIRECTORY_SEPARATOR . $indexFile;

            $exists = true;

            break;
        }
} else {
    // check if the file exists
    $exists = file_exists($real);
}

// return 404 if nothing was found
if (!$exists) {
    http_response_code(404);
    echo "resource not found";

    return true;
}

// check if static files are requested
switch (pathinfo($real, PATHINFO_EXTENSION)) {
    case 'css':
    case 'js':
    case 'ico':
    case 'png':
    case 'jpg':
        return false;
}

require $real;
return true;