Description:
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Simply put, basename() does ot work with Japanese filepaths. If the filename is Japanese only the extension part of the filename is returned. So a filename "/folder/?t?@?C????.txt" resolves to just ".txt". I discovered the problem when performing a basename() on the $_FILES array's 'name' element for uploaded Japanese files, however after testing the bug occurs no matter how you supply the filename.

My PHP environment is running with UTF-8 internal encoding.

The code snippet below illustrates this perfectly.

Reproduce code:
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<?php
// show normal behavior with roman filename
$filename='/myfolder/roman_filename.txt';
echo "The full filename of the romanized file is $filename.\n"; // /myfolder/roman_filename.txt
$basename=basename($filename);
echo "The basename of the romanized file is $basename.\n"; // /roman_filename.txt
// show behavior with Japanese filename
$filename='/myfolder/???{???̃t?@?C????.txt';
echo "The full filename of the Japanese file is $filename.\n"; // /myfolder/???{???̃t?@?C????.txt
$basename=basename($filename);
echo "The basename of the Japanese file is $basename."; // .txt
?>

Expected result:
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The full filename of the romanized file is /myfolder/roman_filename.txt.
The basename of the romanized file is roman_filename.txt.
The full filename of the Japanese file is /myfolder/???{???̃t?@?C????.txt.
The basename of the Japanese file is ???{???̃t?@?C????.txt.

Actual result:
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The full filename of the romanized file is /myfolder/roman_filename.txt.
The basename of the romanized file is roman_filename.txt.
The full filename of the Japanese file is /myfolder/???{???̃t?@?C????.txt.
The basename of the Japanese file is .txt.