web_app.php

<?php
// minimal routing
function web_app($routes, $verb, $path) {

  $verb = strtoupper($verb);
  $path = trim(parse_url($path, PHP_URL_PATH), '/');

  foreach ($routes as $route => $func) {

    @list($req_verb, $req_path) = preg_split('/\s+/', $route, 2);

    if (!$req_verb || !$req_path) {
      header('HTTP/1.1 500 METHOD and PATH required for routes', true);
      exit;
    }

    $req_verb = strtoupper($req_verb);
    $req_path = '@^'.trim($req_path, '/').'$@i';

    if ($verb !== $req_verb || !preg_match($req_path, $path, $caps))
      continue;

    call_user_func_array($func, array_slice($caps, 1));

    exit;
  }

  header('HTTP/1.1 404 Page not found', true);
  exit;
}

// sample use
$a = ['GET /a' => function () { echo "a!"; }];
$b = ['GET /b' => function () { echo "b!"; }];
$c = ['GET /c' => function () { echo "c!"; }];

web_app(
  $a + $b + $c,
  $_SERVER['REQUEST_METHOD'],
  $_SERVER['REQUEST_URI']
);
?>