feat(router): add error boundary with exception callback

src/Libs/Router.php

@@ -50,7 +50,7 @@ class Router
      *
      * @var callable $notFoundCallback
      */
-    public static $notFoundCallback = 'Libs\Router::defaultNotFound';
+    public static $notFoundCallback = Router::defaultNotFound(...);
 
     /**
      * Default callback function for when
@@ -64,6 +64,54 @@ class Router
         echo '<h2 style="text-align: center;margin: 25px 0px;">Error 404 - Page Not Found</h2>';
     }
 
+    /**
+     * The callback function to be executed when the router's boundary
+     * catches an exception thrown anywhere in the matched route chain.
+     * It receives the exception as a named argument: the handler must
+     * declare its parameter as $exception (typeable as \Throwable).
+     *
+     * @var callable $exceptionCallback
+     */
+    public static $exceptionCallback = Router::defaultException(...);
+
+    /**
+     * Default callback function for exception responses.
+     *
+     * The HTTP status comes from the exception's code only when it is an
+     * integer in the 400-599 range; anything else (0, arbitrary codes, the
+     * SQLSTATE string carried by PDOException) answers 500. The body is
+     * negotiated from the request's Accept header: JSON when the client asks
+     * for application/json, plain text otherwise. The trace is serialized
+     * from getTraceAsString() because json_encode() of a Throwable yields an
+     * empty object: its properties are protected.
+     *
+     * @param \Throwable $exception
+     *
+     * @return void
+     */
+    public static function defaultException(\Throwable $exception): void
+    {
+        $code = $exception->getCode();
+        http_response_code(is_int($code) && $code >= 400 && $code <= 599 ? $code : 500);
+
+        $accept = $_SERVER['HTTP_ACCEPT'] ?? '';
+
+        if (str_contains($accept, 'application/json')) {
+            header('Content-Type: application/json');
+            print(json_encode([
+                'error'     => $exception->getMessage(),
+                'exception' => get_class($exception),
+                'file'      => $exception->getFile(),
+                'line'      => $exception->getLine(),
+                'trace'     => explode(PHP_EOL, $exception->getTraceAsString()),
+            ]));
+
+            return;
+        }
+
+        print($exception->getMessage() . PHP_EOL . $exception->getTraceAsString());
+    }
+
     /**
      * __construct
      */
@@ -363,12 +411,19 @@ class Router
     /**
      * Applies the route configuration.
      *
+     * This method is the framework's error boundary: any \Throwable thrown
+     * while running the matched route's callback chain, while printing the
+     * returned data, or while resolving the not-found callback is caught
+     * here and rendered through $exceptionCallback. A failing handler is
+     * deliberately left uncaught (double failure falls back to PHP itself).
+     *
      * @param string|null $path (optional) Path to use. If not defined, it detects the current path.
      *
      * @return void
      */
     public static function apply(?string $path = null): void
     {
+        try {
             $path    = $path ?? static::currentPath();
             $routers = match ($_SERVER['REQUEST_METHOD']) { // Selects an array of routers based on the method
                 'POST'   => static::$post,
@@ -408,5 +463,8 @@ class Router
 
             // If no router matches, call $notFoundCallBack
             Synapsis::resolve(static::$notFoundCallback);
+        } catch (\Throwable $exception) {
+            Synapsis::resolve(static::$exceptionCallback, ['exception' => $exception]);
+        }
     }
 }