Unexpected output with zlib_encode()

Question

I'm trying to encode a chunk of binary data with PHP in the same way zlib's compress2() function does it. However, using zlib_encode(), I get the wrong encoded output. I know this because I have a C program that does it (correctly). When I compare the output (using a hex editor) of the C program against that of the PHP script below, I notice it doesn't match at all.

My question I guess is, does this really compress in the same way zlib's compress2() function does?

<?php
$filename = 'C:\data.bin';
$in = fopen($filename, 'rb');
$data = fread($in, filesize($filename));
fclose($in);

$data_dec = zlib_decode($data);
$data_enc = zlib_encode($data_dec, ZLIB_ENCODING_DEFLATE, 9);
?>

The compression level is correct, so it should match with the C program's encoded output. Is there a bug somewhere perhaps.. ?

Answer 1

Yes, zlib_encode() (with the default arguments), and uncompress() are compatible, and compress2() and zlib_decode() are compatible.

The way to check is not to compare compressed output. Check by decompressing with uncompress() and zlib_decode(). There is no reason to expect that the compressed output will be the same, and it does not need to be. All that matters is that it can be losslessly decompressed on the other end.