Description:
------------
fflush file make strange result in phar files.

this occurs on PHP 7.4 also, tested in windows and linux

use php -n -dphar.readonly=0 test.php to test

Test script:
---------------
<?php

// make up the phar
$phar = new \Phar(__DIR__ . DIRECTORY_SEPARATOR . 'test.phar');
$phar->addEmptyDir('dir'); // may be irrelative
$phar->stopBuffering();

//$phar_uri = __DIR__ . DIRECTORY_SEPARATOR . 'file';
$phar_uri = 'phar://' . __DIR__ . DIRECTORY_SEPARATOR . 'test.phar' . DIRECTORY_SEPARATOR . 'dir/file';

// make a file
$f = fopen($phar_uri, 'w+');
fwrite($f, 'this is content ');
fflush($f); // comment out this, things will be ok
fwrite($f, "of file.\n");
fclose($f);

// read it
$f = fopen($phar_uri, 'r');
$content = fread($f, 4096);
if ($content !== "this is content of file.\n") {
    throw new Exception("bad file context: $content vs 'this is content of file.\n'");
} else {
    echo "OK\n";
}

Expected result:
----------------
OK

Actual result:
--------------
bad file context with strange things