They were suggestions for you to look at, sorry. I don't see where it hardcodes the realm. I get

1..2
test:http://jigsaw.w3.org/HTTP/Digest/: => foo/bar
test:http://jigsaw.w3.org/HTTP/Digest/: => /
test:http://jigsaw.w3.org/HTTP/Digest/: => guest/
test:http://jigsaw.w3.org/HTTP/Digest/: => guest/guest
ok 1
ok 2
[download]

test doesn't appear anywhere in the code and the docs for get_basic_credentials say The arguments passed in is the $realm provided by the server, the $uri requested and a boolean flag to indicate if this is authentication against a proxy server. so isn't this what you want?