I've discovered that when I upgrade PHP on my server from 5.4 to 5.5, all SilverStripe sites on the server return a 500 Server Error until a dev/build is run on each one.

The Apache error log shows the error is:

[Sun Jul 19 12:57:57.279194 2015] [core:error] [pid 215354] [client 82.29.160.195:49790] End of script output before headers: main.php

I'm about to upgrade my main server's PHP, and don't really want 50+ sites to be failing until a dev/build is run, so I'm trying to get to the bottom of what's causing this and exactly why a dev/build fixes it.

I've tried taking a database dump before and after the dev/build and comparing them, but there were no differences at all.

I've found that if I downgrade from PHP 5.5 to 5.4 this triggers the same problem, and dev/build is required again.

What else is dev/build changing that could be fixing this issue?

I've narrowed it down a little further. Seems that clearing the /tmp/silverstripe-cache-home-USERNAME-public_html/USERNAME/staticmanifest directory after the PHP upgrade stops the error for that site.

I compared the files inside this directory both before and after the issue and the only things that changed within them was the "i" number:

Anyone able to advise what this "i" number is? And how I could stop it from changing so that I won't need to clear the cache for all my SilverStripe sites after upgrading to PHP 5.5?