Adds explanatino for ::flatten_array(); optimized array_is_list pollyfill

Author: jtreminio

modules/openapi-generator/src/main/resources/php/ObjectSerializer.mustache

@@ -622,10 +622,16 @@ class ObjectSerializer
     }
 
     /**
+     * Flattens an array of Model object and generates an array compatible
+     * with formdata - a single-level array where the keys use bracket
+     * notation to signify nested data.
+     *
+     * @param \ArrayAccess|array $source
+     *
      * credit: https://github.com/FranBar1966/FlatPHP
      */
     private static function flatten_array(
-        array $source,
+        mixed $source,
         array &$destination,
         string $start = '',
     ) {
@@ -638,11 +644,28 @@ class ObjectSerializer
             'suffix-list-end' => true,
         ];
 
-        // array_is_list only in PHP >= 8.1
+        if (!is_array($source)) {
+            $source = (array) $source;
+        }
+
+        /**
+         * array_is_list only in PHP >= 8.1
+         *
+         * credit: https://www.php.net/manual/en/function.array-is-list.php#127044
+         */
         if (!function_exists('array_is_list')) {
-            function array_is_list(array $arr)
+            function array_is_list(array $array)
             {
-                return $arr === [] || (array_keys($arr) === range(0, count($arr) - 1));
+                $i = -1;
+
+                foreach ($array as $k => $v) {
+                    ++$i;
+                    if ($k !== $i) {
+                        return false;
+                    }
+                }
+
+                return true;
             }
         }