如何保存PHP图像处理的输出

I'm not sure whether or not this function does what I intend it to do, but what I need is to create a thumbnail of an image in the folder.

$filename = '../images/' . $new_name;

header('Content-Type: image/jpeg');

list($width, $height) = getimagesize($filename);

$new_width = $width / $height;

$new_height = 250 / $new_width;

$new_height = round($new_height);

$image_p = imagecreatetruecolor(250, $new_height);
$image_s = imagecreatefromjpeg($filename);
imagecopyresampled($image_p, $image_s, 0, 0, 0, 0, 250, $new_height, $width, $height);

imagejpeg($image_p, null, 100);

So I need to save the final result to my folder, but not overwrite the original image. I found this function on the http://php.net/, but I have no idea as to how to save the image.

imagejpeg($image_p, "path/to/dir/image.jpg", 100);

And that is all.