Description:
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From manual page: http://www.php.net/function.rename
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"Attempts to rename oldname to newname, moving it between directories if necessary. If newname exists, it will be overwritten."

If newname is read-only, Windows will return an access denied error while Linux will overwrite the file, as expected. I suggest adding the following to the above description:

"On Windows, newname must be writable to be overwritten. On Linux, the containing directory must be writable, regardless of the permissions on newname."

If that is too wordy or detailed, I suggest just adding the first sentence.

See also: https://bugs.php.net/bug.php?id=71035

Thanks.