how do you send a lot of data between scripts

How can I send a lot of data from foo.php to bar.php ?

This might happen if foo.php echo some text and bar.php returns an image
that should be displayed below the text that was echo:ed, and foo.php must
send a lot of data to bar.php.

Yes, I know, I can draw the picture in foo.php, and save it to a file, but
what if that is not an option here. Is there a way to send the data without
creating a huge URL?
.