web_app.php

<?php
/**
* Basic dispatch function, uses array structured as [$verb => [$path => $route_fn, ...]]
* calling $route_fn if $path/$verb match the arguments passed 
*
* @param $path_condition regex for matching paths (must use suitable deliminator, eg. @)
*
* @throws Exception when path/route function missing (programmer error)
* @return bool  true for successful route, false for no route/404
*/
function web_app(array $routes, $verb, $path, $path_condition = '@%s@i') {
  $path = trim(parse_url($path, PHP_URL_PATH), '/');

  if(empty($routes[$verb])) {
    return false;
  }

  foreach ($routes[$verb] as $route_path => $route_fn) {
    if(empty($route_path) || empty($route_fn)) {
      throw new Exception('Route Path and Function requred!');
    }

    if (preg_match(sprintf($path_condition, trim($route_path, '/')), $path, $captured)) {
      call_user_func_array($route_fn, array_slice($captured, 1));
      return true;
    }
  }

  return false;
}

$get = ['GET' => [
         '/a' => function () { echo 'a!'; },
         '/b' => function () { echo 'b!'; }
       ]];

$post = ['POST' => [
          '/c' => function () { echo 'c!'; }
        ]];

//$_SERVER['REQUEST_METHOD'] = 'GET';
//$_SERVER['REQUEST_URI']    = '/a';

if(!web_app($post + $get, strtoupper($_SERVER['REQUEST_METHOD']), $_SERVER['REQUEST_URI'])) {
  header('HTTP/1.1 404 Page not found');
}