Merge pull request #1468 from Parship12/fix/php-flatten-name-collision

fix: use array_merge in flatten() to handle duplicate keys correctly

Author: ChiragAgg5k

templates/php/src/Client.php.twig

@@ -354,7 +354,7 @@ class Client
             $finalKey = $prefix ? "{$prefix}[{$key}]" : $key;
 
             if (is_array($value)) {
-                $output += $this->flatten($value, $finalKey); // @todo: handle name collision here if needed
+                $output = array_merge($output, $this->flatten($value, $finalKey));
             }
             else {
                 $output[$finalKey] = $value;