Description:
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"rename — Renames a file or directory"

"Attempts to rename oldname to newname, moving it between directories if necessary. If newname exists, it will be overwritten."

Rename will not overwrite a directory that already exists.

Test script:
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mkdir('a/', 0777);
mkdir('b/', 0777);
rename('a/', 'b/');

Actual result:
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Warning: rename(a/,b/): File exists