PHP Notice: Undefined offset

<?php
$group_id     = '1456456546'; 
$access_token = 'token4535353'; 
$message      = 'Hello World'; 
 
 
$folder_mas = array('images/photos1','images/photos2','images/photos3');
$images = array();
 
$folder = $folder_mas[rand(0,2)];
$all_files = scandir($folder);
while ($i++ < sizeof($all_files)){
    
    if (!strstr($all_files[$i],".png") and !strstr($all_files[$i],".jpg") and
    !strstr($all_files[$i],".gif")) continue;
    array_push($images, $all_files[$i]);
}
$img_random = $images[rand(0,sizeof($images)-1)];
$img_src = $folder."/".$img_random;
 
 
 
 
$res = json_decode(file_get_contents(
    'https://api.vk.com/method/photos.getWallUploadServer?group_id='
    . $group_id . '&access_token=' . $access_token
));
 
if (!empty($res->response->upload_url)) {
    
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $res->response->upload_url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_POSTFIELDS, array('photo' => '@' . $img_src));
 
    
    curl_setopt($ch, CURLOPT_POSTFIELDS, array('photo' => new CurlFile($img_src)));
 
    $res = json_decode(curl_exec($ch));
    curl_close($ch);
 
    if (!empty($res->server)) {
        
        $res = json_decode(file_get_contents(
            'https://api.vk.com/method/photos.saveWallPhoto?group_id=' . $group_id
            . '&server=' . $res->server . '&photo='
            . stripslashes($res->photo) . '&hash='
            . $res->hash . '&access_token=' . $access_token
        ));
 
        if (!empty($res->response[0]->id)) {
            
            $params = array(
                'access_token' => $access_token,
                'owner_id'     => '-' . $group_id,
                'from_group'   => '1',
                'message'      => $message,
                'attachments'  => $res->response[0]->id
            );
 
            file_get_contents(
                'https://api.vk.com/method/wall.post?' . http_build_query($params)
            );
        }
    }
}
unlink($img_src);
?>

$i = 0;
 
while ($i < sizeof($all_files)){
    if (!strstr($all_files[$i],".png") and !strstr($all_files[$i],".jpg") and
    !strstr($all_files[$i],".gif")) continue;
    array_push($images, $all_files[$i]);
$i++;
}

if (strstr($all_files[$i],".png") || strstr($all_files[$i],".jpg") || strstr($all_files[$i],".gif"))
    array_push($images, $all_files[$i]);