The documentation to dirname() states:
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Note: In PHP 4.0.3, dirname() was fixed to be POSIX-compliant. Essentially, this means that if there are no slashes in path , a dot ('.') is returned, indicating the current directory. Otherwise, the returned string is path with any trailing /component removed. Note that this means that you will often get a slash or a dot back from dirname() in situations where the older functionality would have given you the empty string.
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This is not true for 4.2.2, for the root directory it (still or again?) returns the empty string.