I don't believe PHP's ini_read caches anything. Sure, your OS cache is going to have that file in, but PHP still has to do the fopen(), fread(), parse, fclose() dance on every page hit. This does not happen with an opcode-cached source file.
I have actually benchmarked this. Amortized across billions of page hits per month it produces a notable saving.
Kudos points out that you could use APC - this is absolutely true but IMO it basically equates to doing the same thing in a more complex way.
> Configuration stored in a PHP file is bad. Changing working code simply because you want to add a new slave is a horrible idea. It would be like having to recompile Apache from source every time you want to add a new vhost because all the entries are written in C.
This is a false comparison between interpreted and compiled languages. Not to mention that plenty of C programs use #defines for configuration. Varnish even translates its configuration language into C and dynamically loads it as a module, which provides significant flexibility.
In a dynamic language, and where you trust the person doing the configuration, there's no reason why your configuration shouldn't be in a source file. Both Django and Rails do it this way.