It turns out basename does a bit more than just splicing the string at the last slash, because it's locale aware. In my case I was dealing with a multi-byte UTF-8 string. It took me quite some time figuring out what was going on, because I was testing from the console which had the en_US.UTF-8 locale, and the bug was appearing on Apache, which defaults to the C locale.

He includes an example snippet of code showing how it can work with both the default (well, for Apache anyway) of the "C" locale versus the "UTF-8" locale and return different results for the same urldecoded information.

symfony and other applications rely on the server's PATH_INFO being set properly. Unfortunately, I use a nonstandard server that doesn't natively support CGI [...] but I can't figure out how to do a urldecode in my configuration.

To get around the issue, he created a file he prepended to each request (via auto_prepend_file) that took the value and urldecoded it to put it in another $_SERVER value.