$json_string = file_get_contents('http://api.vk.com/method/wall.get?v=5.7&domain=verstka_html');
$json = json_decode($json_string, true);
$all_posts = $json['response'];
$photoUrl = array();
foreach ($all_posts['items'] as $post) {
  if (!empty($post['attachments'])) {
    foreach ($post['attachments'] as $postAttachment) {
      if ($postAttachment['type'] == 'photo') {
        $photoUrl[] = $postAttachment['photo'];
      }
    }
  }
}

foreach ($photoUrl as $item) {
  if (!empty($item['photo_807'])) {
    return $item['photo_807'];
  }
}
return '';