Description:
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The gzencode and gzwrite functions, which implement RFC 1952 (http://www.gzip.org/zlib/rfc-gzip.html), don't read/write the mtime header.
While this is not necessary for gzip compliance (http://www.gzip.org/zlib/rfc-gzip.html#id-compliance) adding a function to read/write the mtime header when compressing or reading compressed files would be very useful in many cases.

I currently use custom code to read/write the gzip mtime header.

Read:

$gzmtime = end(unpack('V', file_get_contents($gzfile, null, null, 4, 4)));

Write:

$r = fopen($gzfile, 'r+');
fseek($r, 4);
fwrite($r, pack('V', $filemtime), 4);
fclose($r);