The most interesting bug in PHP is the showstopper bug in the core of PHP you finally run into after a month of software development just as you are getting ready to ship a brand new product out the door. Specifically, PHP bug #72333, which is in all current versions of PHP. If you aren't familiar with reading C code, it can be extremely hard to follow along with that bug report especially since PHP streams behind-the-scenes are ugly beasts to try to wrap your head around (mine's still spinning and I wrote the bug report). In short, the problem is a combination of non-blocking mode with SSL sockets when calling SSL_write() with different pointers in 'ext\openssl\xp_ssl.c'.
The temporary patch in userland is to disable non-blocking mode when writing data - if you can - I'm not so sure I can/should. The correct solution is to fix PHP itself by altering how it interfaces with OpenSSL, which could be as simple as altering a couple of lines of code. I'd submit a patch, but I'm not entirely sure what the correct course of action should be since the problem happens so deep in the code and even my suggested fix might cause the more common case (i.e. blocking sockets) to break. It's kind of rare to need the ability to write tons of data to non-blocking SSL sockets in PHP, so it is not surprising that very few people have run into the issue.
Once you've started reading the actual C source code to PHP, it becomes rather frustrating to see how few people actually read the source code to PHP. This is no more self-evident than the comments section on every documentation page on php.net, GitHub, Stack Overflow, forums, and mailing lists where people make uninformed guesses and subsequently pollute issue trackers and Google search results. I blame a combination of laziness and...wait, no, it's pretty much laziness. You can actually download the source code to PHP here [mind blown]. Instead of just blindly compiling and running PHP, you can actually read the source code [mind blown again].
Of course, that doesn't mean the PHP source code is easy to follow - it is written in C and 80% of the code is basically a hodgepodge of horribleness that exists to deal with cross-platform and third-party library integration issues and various bits of ancient cruft that have stuck around from the very beginning of the language. It would probably look a lot cleaner though if the PHP documentation itself linked to the source code (I opened that ticket too but missed proofreading one sentence - sigh). After all, most people tend to spruce things up when they know guests are coming over to visit.