Explicitly define the order of the execution of deferred callbacks (#21)

src/functions.inc.php

@@ -11,21 +11,14 @@
 
 function defer(?SplStack &$context, callable $callback): void
 {
-    $context = $context ?? new SplStack();
-
-    $context->push(
-        new class($callback) {
-            private $callback;
-
-            public function __construct(callable $callback)
-            {
-                $this->callback = $callback;
-            }
-
-            public function __destruct()
-            {
-                \call_user_func($this->callback);
+    $context ??= new class() extends SplStack {
+        public function __destruct()
+        {
+            while ($this->count() > 0) {
+                \call_user_func($this->pop());
             }
         }
-    );
+    };
+
+    $context->push($callback);
 }