If you own the hardware I imagine much of your PHP/html stuff will be served from the file system cache much of the time so you probably wouldn't see much benefit...
I don't think it would do much there either: If it fits in your left-over RAM, then it's probably in the disk cache. If it doesn't, then you can't create a RAM disk large enough.
It might help with latency for the long-tail of data that isn't used very often and thus maybe replaced in the cache by other data, but on the other hand the OS probably had a reason to replace it and forcing it to stay in RAM might slow everything down.