将图像从一个网站复制到我的网站？

I want to make a website where you you submit a list of external images and receive a zip with all the images in it.

I know how to make zip files with PHP, but I don't know how to save an external file to my server. I would search, but I have no clue how to phrase a question like that.

At a high level...

$externalFile = 'http://example.com/image.jpg';

file_put_contents(basename($externalFile), file_get_contents($externalFile));

Of course, you'll want to handle errors and ensure the URLs are images (use GD or similar).