Fatal error: Call to undefined function json_encode()
I subsequently Googled around and came across a highly-ranked blog article which makes it sound like PHP 5.5 drops support for json_encode because of some silly licensing argument. The truth is, PHP 5.5 does NOT drop support for json_encode. For a much better and much clearer explanation, I recommend reading this article.
Now, enough of the balleyhoo! How do we fix it?
It was was easy! Install the php5-json package and restart Apache. Voilà!
sudo apt-get install php5-json sudo /etc/init.d/apache2 restart
Note that the author of the blog article linked above mentions that re-installing the php5-dev pacakged fixed his woes. While that didn't help me, you may want to give it a try if the above didn't help you.