Why vk api cover download method returns null?

Question

PHP

Good afternoon. Stuck on the step of sending the cover file to the server. I do the following:

$filename = '../web/covers/background-color-explosion-sky-abstact-colors-125-211.jpg';
$url = $this->PhotoUploadServer($group_id);
$photo = $this->UploadPhoto($url, $filename);

Here I note that $url is correct, and file_exist($filename) returns true

public function UploadPhoto($url, $file)
{
        $data = [
            'photo' => new CURLFile($file),
        ];
        $out = $this->request($url, $data);
        return $out;
}

public function request($url, $data = array()) {
        $curl = curl_init();
        $data['access_token'] = $this->token;
        curl_setopt($curl, CURLOPT_URL, $url);
        curl_setopt($curl, CURLOPT_HTTPHEADER,[
            "Content-Type:multipart/form-data"
        ]);
        curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($curl, CURLOPT_CUSTOMREQUEST, 'POST');
        curl_setopt($curl, CURLOPT_POST, true);
        curl_setopt($curl, CURLOPT_POSTFIELDS, $data);
        $out = json_decode(curl_exec($curl), true);

        curl_close($curl);
        return $out;
    }

var_dump($out) = null;

Actually, I can not understand why null and what he does not like?