How do I can post a multiple photos via Facebook API

You can make batch requests as mentioned here: https://stackoverflow.com/a/11025457/1343690

But its simple to loop through your images and publish them directly.

foreach($photos as $photo)
{
       //publish photo
}

Edit:

(regarding grouping of photos on wall)

This grouping is done by facebook automatically if some photos are uploaded into the same album.

Currently you cannot create an album in a group via Graph API - it is not supported (as of now), see this bug.

But you can do this - create an album manually, then get the album_id by-
\GET /{group-id}/albums, then use the the code with album_id instead of group_id-

foreach($photos as $photo){
   $facebook->api("/{album-id}/photos", "POST", array(
      'access_token=' => $access_token,
      'name' => 'This is a test message',
      'url' => $photo
      )
   );
}

---

I've tested it, see the result-

You can now publish multiple images in a single post to your feed or page:

For each photo in the story, upload it unpublished using the {user-id}/photos endpoint with the argument published=false.

You'll get an ID for each photo you upload like this:

{
  "id": "10153677042736789"
}

Publish a multi-photo story using the {user-id}/feed endpoint and using the ids returned by uploading a photo

 $response = $facebook->api("/me/feed", 'POST',
  array(
    'access_token=' => $access_token,
    'message' => 'Testing multi-photo post!',
    'attached_media[0]' => '{"media_fbid":"1002088839996"}',
    'attached_media[1]' => '{"media_fbid":"1002088840149"}'
  )
);

Source: Publishing a multi-photo story