Router for Drupal 8 when using PHP InBuilt Webserver in Development

router.php

<?php

$file = pathinfo($_SERVER["SCRIPT_FILENAME"]);

$mimeTypes = [
    'css' => 'text/css',
    'js'  => 'application/javascript',
    'jpg' => 'image/jpg',
    'png' => 'image/png',
    'map' => 'application/json',
    'svg' => 'image/svg+xml'
];

if(array_key_exists($file['extension'], $mimeTypes)) {

    $path = $_SERVER["DOCUMENT_ROOT"] . $_SERVER['REQUEST_URI'];

    if (is_file($_SERVER["SCRIPT_FILENAME"])) {
        header("Content-Type: {$mimeTypes[$file['extension']]}");
        readfile($_SERVER["SCRIPT_FILENAME"]);
        exit;
    }
} else {
    require 'index.php';
}