Best way to get the contents of a local file in a server variable

My script acts on a file (approx. 100k) offered by the user. Actually, it
acts on a variable that should have the contents of that file in it. I
could try to let users upload that file and then read that file into a
variable, but before I figure out how to do that: is this unnecessary
overhead? Is it possible to do it without creating a file on the server
filesystem and do it completely within the PHP framework? What method
would you recommend?
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