Create Image thumb while uploading

if(!empty($_FILES)){
                        $img = explode('.', $_FILES['thumb_img']['name']);
                        $img_path = $img_dir .$img[0].time().'.'.$img[1];
                        $img_thumb = $img_dir .$img[0].time().'_thumb.'.$img[1];
                        $extension = strtolower($img[1]);
                        if(in_array($extension , array('jpg','jpeg', 'gif', 'png', 'bmp'))){
 
                                //---------- To create thumbnail of image---------------
                                if($extension=="jpg" || $extension=="jpeg" ){
                                        $src = imagecreatefromjpeg($_FILES['thumb_img']['tmp_name']);
                                }
                                else if($extension=="png"){
                                        $src = imagecreatefrompng($_FILES['thumb_img']['tmp_name']);
                                }
                                else{
                                        $src = imagecreatefromgif($_FILES['thumb_img']['tmp_name']);
                                }
                                list($width,$height)=getimagesize($_FILES['thumb_img']['tmp_name']);
                                $newwidth=151;
                                $newheight=151;//($height/$width)*$newwidth;
                                $tmp=imagecreatetruecolor($newwidth,$newheight);
                                imagecopyresampled($tmp,$src,0,0,0,0,$newwidth,$newheight, $width,$height);
                                imagejpeg($tmp,$img_thumb,100);
                                //--------------    Ends   -----------
 
                                move_uploaded_file($_FILES['thumb_img']['tmp_name'], $img_path);
                                $blog_thumb = get_site_option('rt_blog_thumb');
                                $blog_thumb[$blog_id] = basename($img_thumb);
                                update_site_option('rt_blog_thumb', $blog_thumb);
                        }
                        else echo 'Invalid file';
                }