You can use any single file, including a JSON file, like this:
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Lock it somehow (google PHP file locking, it’s possibly as simple as adding a parameter to file open function or changing function name to locking version).
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Read the data from file and parse it to internal data stucture.
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Optionally modify the data in internal data structure.
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If you modified the data, truncate the file to 0 length and write new data to it.
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Unlock the file as soon as you can, other requests may be waiting…
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You can keep using the data in internal structures to render the page, just remember it may be out-dated as soon as you release the file lock and other HTTP request can modify it.
Also, if you modify the data from user’s web form, remember that it may have been modified in between. Like, load page with user details for editing, then other user deletes that user, then editer tries to save the changed details, and should probably get error instead of re-creating deleted user.
Note: This is very inefficient. If you are building a site where you expect more than say 10 simultaneous users, you have to use a more sophisticated scheme, or just use existing database… Also, you can’t have too much data, because parsing JSON and generating modified JSON takes time.
As long as you have just one user at a time, it’ll just get slower and slower as amount of data grows, but as user count increases, and more users means both more requests and more data, things start to get exponentially slower and you very soon hit limit where HTTP requests start to expire before file is available for handling the request…
At that point, do not try to hack it to make it faster, but instead pick some existing database framework (SQL or nosql or file-based). If you start hacking together your own, you just end up re-inventing the wheel, usually poorly :-). Well, unless it is just programming exercise, but even then it might be better to instead learn use of some existing framework.