Description:
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Phar::decompressFiles marks the files in the PHAR as not compressed but when you extract the files from the PHAR they will be still compressed.

This is the same exact issue as https://bugs.php.net/bug.php?id=76584 just with Phar::decompressFiles instead of PharFileInfo::decompress.

Test script:
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<?php
file_put_contents("test.txt", "This is a test file.");
$phar = new Phar("test.phar");
$phar->addFile("test.txt");
$file = $phar["test.txt"];
var_dump($file->compress(Phar::GZ)); //true (success)
var_dump($file->isCompressed()); //true (the file is compressed)
var_dump($phar->decompressFiles()); //true (success)
var_dump($file->isCompressed()); //false (the file should not be compressed anymore)
var_dump($phar->extractTo("test")); //true (success but the extracted file in the folder is compressed)