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

Author: jtreminio

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

@@ -605,4 +605,81 @@ class ObjectSerializer
 
         return $qs ? (string) substr($qs, 0, -1) : '';
     }
+
+    /**
+     * 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(
+        mixed $source,
+        array &$destination,
+        string $start = '',
+    ) {
+        $opt = [
+            'prefix'          => '[',
+            'suffix'          => ']',
+            'suffix-end'      => true,
+            'prefix-list'     => '[',
+            'suffix-list'     => ']',
+            'suffix-list-end' => true,
+        ];
+
+        if (!is_array($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 $array)
+            {
+                $i = -1;
+
+                foreach ($array as $k => $v) {
+                    ++$i;
+                    if ($k !== $i) {
+                        return false;
+                    }
+                }
+
+                return true;
+            }
+        }
+
+        if (array_is_list($source)) {
+            $currentPrefix    = $opt['prefix-list'];
+            $currentSuffix    = $opt['suffix-list'];
+            $currentSuffixEnd = $opt['suffix-list-end'];
+        } else {
+            $currentPrefix    = $opt['prefix'];
+            $currentSuffix    = $opt['suffix'];
+            $currentSuffixEnd = $opt['suffix-end'];
+        }
+
+        $currentName = $start;
+
+        foreach ($source as $key => $val) {
+            $currentName .= $currentPrefix.$key;
+
+            if (is_array($val) && !empty($val)) {
+                $currentName .= "{$currentSuffix}";
+                self::flatten_array($val, $destination, $currentName);
+            } else {
+                if ($currentSuffixEnd) {
+                    $currentName .= $currentSuffix;
+                }
+                $destination[$currentName] = self::toString($val);
+            }
+
+            $currentName = $start;
+        }
+    }
 }