There are situations where a string instead needs to be used, and for these purposes the data: stream wrapper is used. Initially I thought it was only possible to encode the actual string in base64, which I didn't like because of the added footprint. [...] Quickly checking out the rfc, it turns out that ';base64' can be omitted to just pass along the raw data, which makes a lot more sense in the context of PHP.

His example takes in an example string and pushes it back out the other side after base64 encoding and decoding it. Davey Shafik found a use for it in avoiding an eval call.