Description:
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"zlib.inflate" filters appear to buffer all previously-read data, as exhibited by the attached code, until the file is closed.

It is unclear from my testing if the data being kept in memory is the original or inflated copy.

Test script:
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<?php
// Generate "data.gz" file with:
// dd if=/dev/urandom of=data count=$((8*1024*1024)) iflag=count_bytes; gzip data

function PrintMem($message) { echo $message, ": ", (memory_get_usage() / 1024 / 1024) . " MB\n"; }

$fp = fopen("data.gz", 'rb');
$filter = stream_filter_append($fp, "zlib.inflate", STREAM_FILTER_READ, array( 'window' => 31 ));

PrintMem("before read");
stream_get_contents($fp);
PrintMem("after read");

stream_filter_remove($filter);
fclose($fp);
PrintMem("after close");
?>

Expected result:
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I expect that very little memory is used.

It should be possible to read smaller blocks at a time from a very large gzipped files without buffering the entire content of what has been read so far, but that does not seem to be the case.

Actual result:
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OUTPUT from the above script:

before read: 0.44554138183594 MB
after read: 8.4377517700195 MB
after close: 0.37459564208984 MB