public function verify_img($file){ // you should pass the file instead of just the $_FILES['file']['name']

    $extension=strtolower(substr($file,strpos($file,'.')+1));
    $size=$_FILES['file']['size'];
    $max_size=1000000;
    $type=$_FILES['file']['type'];
    if($extension=='jpg'||$extension=='jpeg'||$extension=='png'&& $type=='image/jpeg'&& $type=='image/jpg'&& $type=='image/png')
             {
                 if($size<=$max_size)
                 {
                    return "ok";
                 }

                 else{

                  return "size_large";
                 }
             }

    else {

      return "invalid_format";
    }

// this is what I will do
    public function verify_img($file){

    $type=$file['type'];
    $size=$file['size'];
    $name=$file['name'];
    $data=$file['tmp_name'];
    $extension=strtolower(substr($name,strpos($name,'.')+1));
    $max_size=1000000;
    if($extension=='jpg'||$extension=='jpeg'||$extension=='png'&& $type=='image/jpeg'&& $type=='image/jpg'&& $type=='image/png')
             {
                 if($size<=$max_size)
                 {
                    return "ok";
                 }

                 else{

                  return "size_large";
                 }
             }

    else {

      return "invalid_format";
    }

// to use it

    $file = $_FILES['upload'];
    $obj->verify_mg($file);