PHP class Router private static defined false shows if controller and

<?PHP
  
  class Router {
    private static
      $defined     = false,    // shows if controller and action was already set
      $args        = array(),  // contains arguments passed through request
      $args_keys   = array(),  // contains names of arguments passed through request
      $args_values = array(),  // contains values of arguments passed through request
      $routes      = array(),  // contains named routes
      $link_args   = array();  // contains arguments sent to a get_link method
    
    private function __construct() {
      // nothing to do here
    }
    
    // sets GET route
    static public function GET($path, $route, $name) {
      self::add_route($path, $name);
      self::request  ($path, $route, 'GET');
    }
    
    // sets POST route
    static public function POST($path, $route, $name) {
      self::add_route($path, $name);
      self::request  ($path, $route, 'POST');
    }
    
    // sets home route
    static public function HOME($route, $name) {
      self::GET('/', $route, $name);
    }
    
    // sets default route
    static public function SET($route) {
      if ( self::$defined )
        return;
      self::define_route($route);
    }
    
    // returns args passed through request
    static public function args() {
      return self::$args;
    }
    
    // returns named link
    static public function get_link($name, $args = array()) {
      if ( !isset(self::$routes[$name]) )
        die("Router: route with name `$name` not found.");
      $path = self::$routes[$name];
      self::$link_args = $args;
      return $url.preg_replace_callback('#/:([^/]+)#iu', 'self::link_gen', $path);
    }
    private static function link_gen($matches) {
      return '/'.array_shift(self::$link_args);
    }
  
    // handles request
    static private function request($path, $route, $method) {
      if ( self::$defined )
        return;
      $path = rtrim($path,'/');
      $request = preg_replace('#(\?.*)#','',$_SERVER['REQUEST_URI']);
      $request = rtrim($request,'/');
      $modified_path = preg_replace('#/:([^/]+)#iu','/([^/]+)',$path);
      if ( preg_match("#^$modified_path$#iu", $request) && $_SERVER['REQUEST_METHOD'] == $method ) {
        self::define_args($path, $request);
        self::define_route($route);
      }
    }
    
    // adds named route
    static private function add_route($path, $name) {
      if ( !preg_match('#^[a-z_][a-z0-9_]*$#iu',$name) )
        die("Router: route name should contain only letters, numbers and underscores and should not start with a number. You've endered: `$name`.");
      self::$routes[$name] = $path;
    }
    
    // extracts args from request
    static private function define_args($path, $route) {
      $modified_path = preg_replace('#/:([^/]+)#iu','/:([^/]+)',$path);
      $modified_route = preg_replace('#/:([^/]+)#iu','/([^/]+)',$path);
      preg_replace_callback("#^$modified_path$#",  'self::set_keys',   $path);
      preg_replace_callback("#^$modified_route$#", 'self::set_values', $route);
      if ( count(self::$args_keys) )
        self::$args = array_combine(self::$args_keys,self::$args_values);
      else
        self::$args = array();
    }
    static private function set_keys($matches) {
      array_shift($matches);
      self::$args_keys = $matches;
    }
    static private function set_values($matches) {
      array_shift($matches);
      self::$args_values = $matches;
    }
    
    // sets controller and action for matched route
    static private function define_route($route) {
      list($controller, $action) = explode('#', $route);
      if ( !defined('controller') && !defined('action') ) {
        define('controller', $controller);
        define('action', $action);
      }
      self::$defined = true;
    }
  }