Is there a variable scope that is accessible anywhere in PHP?

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I'd like to create something like a very basic chat application. I don't want to use a database, since it'd cause a heavy load on an already strained db. I also don't want to use a flat file, because it have a feeling that it'd become a mess or that it'll have lots of read/writes... So, I'm wondering if there is a way to have a variable that is accessible in any file and at any time.