Description:
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When using filenames with special Swedish letters for example "Röd_Statistics.pdf". It's not possible to add files into a zip archive file.

Test script:
---------------
<?PHP
// åöä
ini_set('display_startup_errors',1);
ini_set('display_errors',1);
error_reporting(-1);

$testfile = "C:\\Users\\nikolasengstrom\\Desktop\\Röd_Statistics.pdf";
$outputfile = "C:\\Users\\nikolasengstrom\\Desktop\\Testing1.zip";

$zipfile = new ZipArchive;
$return_code = $zipfile->open($outputfile, ZipArchive::CREATE);
if ($return_code != true) die("Failed to open file: " . $return_code);

$testfile_zip = mb_convert_encoding($testfile, "ISO-8859-1", "UTF-8");
$return_code = $zipfile->addfile($testfile_zip, substr($testfile, (strrpos($testfile, "\\") + 1)));
if ($return_code != true) print("Failed to add file: " . $zipfile->getStatusString());
		
$return_code = $zipfile->close();
if ($return_code != true) die("Failed to close archive: " . $zipfile->getStatusString());
?>