What type of Data Type/Structure is this?

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I’ve seen phpbb use it, wordpress use it, and a few other places from a google search but I can’t seem to find any documentation describing how it works or where it comes from.

It seems like a nice way to store more “abstract” data. Do any of you know what it is, what generates it, or the technical name of it?

I’m embarrassed for not knowing that all these years but I suppose I never did use it before. Shame aside, thank you very very much!

[fphp]serialize[/fphp] and [fphp]unserialize[/fphp]