Description:
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While it is possible to write this by hand using session_set_save_handler or a plugin, it seems like it would be a good idea to build-in saving sessions to a single PHAR Data file. The two technologies would seem to compliment each other fairly well, and it would be helpful as far as file count limitations go, which many shared hosting servers implement. It may also help to stimulate use of PHAR files by developers in other, similar cases.

I have not set up any test environments to see what kind of speed or system usage issues may exist from using PHAR so actively, but as long as the file isn't compressed, it shouldn't be too bad.

It would, additionally, be of some use to re-purpose session.save_path to point to a file rather than a directory. Whether to create a tar/zip option or detect it by file extension is up to you, though I'd suggest creating a session.save_format or save_type or what have you, just in case a user feels like using .sess or .tmp or some other unexpected file extension, or no extension at all. Plus, you already have the 'Phar file format constants' defined. It may also be best to require tar in all instances, if the performance is that superior to zip.