It depends on how often this is called and how intricate you want to make it.

A simple solution would be to download those files through PHP to a local temporary directory. Then you would issue a gzip command on the commandline which you can do through the PHP system() function. After the file is created you can then send that file to the user.

Reading in the output and returning that along with the proper headers to specify the format.

After the file has been received you can then clean out that directory. You can either have the file deleted in PHP after the transfer is complete, or you can setup a crontab on that directory to delete any files that are over “x” amount of time old. Depending on the size of the zip files and how long it takes transfer you can set them to be deleted every 24 hours or so forth.