This is blocking testing of T160481, T162752, T162754, and anything else we do moving forward.

@Samwilson I looked into this for an hour or two last night, but got nowhere :( Would you mind taking a look? What are we doing that could be incompatible with PHP 5.5?

So the error being thrown when purging prod cache was this:

PHP Fatal error:  Call to undefined method Redis::scan() in vendor/symfony/symfony/src/Symfony/Component/Cache/Adapter/RedisAdapter.php on line 214

Which comes about when using the \Redis class instead of \Predis\Client, which it seems was happening because we had cached prod service-constructing code from before (I'm assuming; it's sort of hard to figure out the order of things) we included the Predis library. The cache-clearing system is annoying: it clears the data cache and the code cache, and so if there's an error in the latter that prevents it from finishing, there doesn't seem to be anyway to clear the code cache (short of doing it manually).

So, to fix, manually nuke the cache directories:

$ rm -r  var/cache/*
$ ./bin/console c:c 
 // Clearing the cache for the dev environment with debug true                                                
 [OK] Cache for the "dev" environment (debug=true) was successfully cleared.                                                                                                                                           
$ ./bin/console c:c --env prod
 // Clearing the cache for the prod environment with debug false                                              
 [OK] Cache for the "prod" environment (debug=false) was successfully cleared.                                

And all seems to be okay now. Can you confirm?