Some people argue that, by using PHP to find out the right image filename and then send HTML code that instructs the client's browser to request that image from
the original site and display it, you don't copy anything yourself, only the client's browser copies the file in the client's video memory, which would be necessary anyway. Thus, the argument goes,
no copyright is infringed, since no copies are made by you.
However, even if you don't copy the daily comic directly, you use the original author's work to produce a page that can be seen as a "derivative work". Even if you argue that what you do is just
the same as what any proxy would do, if it where instructed to strip all other content (like advertisements, for example) from the requested page and serve only the image, it is not sure that the
court will follow you in this interpretation.
To avoid even the remote possibility of breaking copyright law, you are well advised to ask the authors' permission, before you use any comics blocks on your site. Some may not agree, in which
case you should respect their wish, but some may do give you permission, perhaps under some easy to accept conditions. For example, to obtain permission to show the UserFriendly cartoon on his site,
Chris wrote the author, Illiad, who kindly agreed under the condition to reproduce the ads that come with it on the original UserFriendly page. This way, everybody's happy!