Note that this function overwrites existing files of the same name.

ZipArchive::FL_ENC_GUESS, ZipArchive::FL_ENC_UTF_8, and ZipArchive::FL_ENC_CP437 affect the $name parameter, not $content

if you try:

<?php
$zip->open("file", ZipArchive::CREATE);
$zip->addFromString("russian_letters/options.xml");
?>

wrong directory will be created.

if you try:

<?php
$zip->addEmptyDir("russian_letters");
?>

All be fine.

in case your string contain Arabic char, ZipArchive::addFromString will show it in wrong format char.
in this case you have to change your string coding from Unicode to Windows-1256 code

$filetxt        = iconv('utf-8','CP1256',$filetxt);
$zip            = new ZipArchive;
$zipFName    = "Tmp.zip";
if ($zip->open($zipFName, ZipArchive::CREATE) === TRUE)
{
    $zip->addFromString($fileName . '.csv',  $filetxt);
    $zip->close();
}

On PHP >5.4, This function will usually create any subfolders inside the ZIP archive.
For instance:

   $zip->addFromString ( 'path/to/file.txt' , $data );

will create the folders "path/", and "path/to/" in addition to placing the newly created file "file.txt" in "path/to/" folder.

Although numFiles will change after overwriting a file, it will be back normal when you ZipArchive::close() and open() it again.
NULL indexes don't persist either. Only the new order is kept. So feel free to overwrite.

Here overwriting works the same as deleting and adding. So it's not necessary to ZipArchive::deleteName() first.

Although this function displaces files of the same name, in actual fact, the original file is blanked and a new entry is added. The numFiles property is incremented.

Example:

File 1: foo
File 2: bar

$zip->addFromString('foo', 'new foo');

File 1:
File 2: bar
File 3: foo