<?php

$group_id     = 'XXX';
$access_token = 'XXXX';
$message      = 'Hello, world!';
$image        = 'test.jpg';

$url = vk('photos.getWallUploadServer', [
    'group_id' => $group_id,
    'v' => '5.85',
    'access_token' => $access_token
])->response->upload_url; // Обращаемся сразу к <response[upload_url]>

if (isset($url))
{
    $upload = json_decode(curl($url, ['photo' => new CURLFile($image)]));

    if (isset($upload->server))
    {
        $save = vk('photos.saveWallPhoto', [
            'group_id' => $group_id,
            'server' => $upload->server,
            'access_token' => $access_token,
            'hash' => $upload->hash,
            'photo' => $upload->photo,
            'v' => '5.85'
        ])->response[0]; // Обращаемся сразу к <response[0]>

        if (isset($save))
        {
            $post = vk('wall.post', [
                'owner_id' => '-'.$group_id,
                'access_token' => $access_token,
                'from_group' => 1,
                'message' => $message,
                'attachments' => 'photo'.$save->owner_id.'_'.$save->id,
                'v' => '5.85'
            ]);

            print_r($post);
        }
    }
}

function vk($method, $params)
{
    return json_decode(curl('https://api.vk.com/method/'.$method, $params));
}

function curl($url, $params = false)
{
    $ch = curl_init($url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);

    if (isset($params))
    {
        curl_setopt($ch, CURLOPT_POST, 1);
        curl_setopt($ch, CURLOPT_POSTFIELDS, $params);
    }

    $upd = curl_exec($ch);
    curl_close($ch);

    return $upd;
}