Howdy readers; happy Friday! It’s been a while since I’ve blogged and I was trying to think about something to write about, so here’s a quick one to share something I learned during the PHP7 migration process.

I installed PHP7 on my local machine (Windows/IIS) and set about making sure our application code worked fine. After a few tweaks, all our tests passed and everything seemed rosey.

However, over the course of the next few days I noticed that IIS was intermittently responding with error 500s on requests for static assets (i.e. CSS/JS and occasinoally images). The problem seemed to disappear when I switched IIS back to using PHP 5.6, even though PHP wasn’t actually involved in the requests for CSS/JS.

I was usually able to make the problem go away by clearing my app cache, restarting IIS and recycling my application pools. However, a couple of days ago I started getting the 500s from PHP requests as well, which gave me a lovely IIS error screen including a FastCGI error code (the helpfully named 0xfffffffe).

The Google rabbit hole eventually lead me to suspecting that OpCache was somehow involved. I enabled the PHP.ini directive opcache.error_log, and sure enough, there was an entry in this for each of the 500s that IIS was dishing out:

Base address marks unusable memory region. Please setup opcache.file_cache and opcache.file_cache_callback directives for more convenient Opcache usage.
Attempt to access invalid address.

It seems like OpCache was having some memory issues, and by default it has no fallback-options for writing to file cache. By setting the following INI directives, it gives OpCache some fallback options and the 500-coded responses disappeared - hooray!