Description:
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The rename function overwrites. This is totally unexpected and unintuitive behavior which could cause data loss, and it is not even documented.

Combined with the fact that the rename function can "rename" across drives, it's obviously a move, not a rename. Then it should be called move, so you can tell what it does.

Test script:
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<?php

file_put_contents('1.txt', 'one');
file_put_contents('2.txt', 'two');

rename('2.txt', '1.txt');

echo '1.txt:' . file_get_contents('1.txt');
echo '2.txt:' . file_get_contents('2.txt');

Expected result:
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1.txt:one
2.txt:two

plus an error that the name exists

Actual result:
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1.txt:two
2.txt:

plus an error that 2.txt does not exist