I hate "Misc" sections as much as you do, but I hope this chapter has been a gold mine for your imagination.
If you're writing any sort of command-line app, learning process control fully will yield big results. Get your process control skills right and you'll be fine.
Similarly, the POSIX functions are a great way to let you interact with the OS, which is also very popular for CLI scripts.
SPL and the reflection API are quite hard, but worth the effort. SPL can really help you clean up your scripts with some new, advanced functionality, and reflection can be thought of as a PHP manual for the rest of your code.
Browser detection is about more than just looking at the user agent - through the get_browser() function you can check for all sorts of advanced functionality and make use of it where available.
"Arbitrary-precision mathematics" are not dirty words.
Spell checking in multiple languages is a cinch in PHP, but with a little thought you can make it much more exciting.
Templates mean your designers can work independent of your coders, which lets both groups do what they do best. Having said that, they don't magically make things better. There are good template systems out there already (most notably Smarty), but you can create your own if your needs are simple.
Copyright ©2015 Paul Hudson. Follow me: @twostraws.