zlib.deflate and zlib.inflate

zlib.deflate (compression) and zlib.inflate (decompression) are implementations of the compression methods described in » RFC 1951. The deflate filter takes up to three parameters passed as an associative array. level describes the compression strength to use (1-9). Higher numbers will generally yield smaller payloads at the cost of additional processing time. Two special compression levels also exist: 0 (for no compression at all), and -1 (zlib internal default -- currently 6). window is the base-2 log of the compression loopback window size. Higher values (up to 15 -- 32768 bytes) yield better compression at a cost of memory, while lower values (down to 9 -- 512 bytes) yield worse compression in a smaller memory footprint. Default window size is currently 15. The zlib.deflate filter implements the compression methods DEFLATE, ZLIB and GZIP depending on the value of the window parameter. The 4 lower bits of the window parameter set the size of the internal “history buffer” used, being the base-2 logarithm of its size in a range from 8 up to 15. The meaning of the others bits of the window parameter can be set as described below.

• DEFLATE (» RFC 1951) is a raw compression algorithm without header, without checksum. It is performed when the window parameter is set in the range from -9 up to -15. This compression algorithm is the base for all the formats generated by the zlib.deflate filter. The corresponding functions that operate directly on strings are gzdeflate() and gzinflate().

• ZLIB (» RFC 1950) applies the DEFLATE algorithm and adds a 2 bytes header and 4 bytes trailer containing the Adler32 checksum of the uncompressed data in big-endian byte order:

  ZLIB = ZLIBHEADER(2B)  DEFLATE  ADLER32(4B)

  The 2 bytes header, read as 16 bit unsigned number in big-endian order, must be multiple of 31. This format is generated when the window parameter is set in the range from 8 up to 15. The corresponding functions that operate directly on strings are gzcompress() and gzuncompress().

• GZIP (» RFC 1952) applies the DEFLATE algorithm adding an header and a trailer with the CRC32 checksum of the uncompressed data and their length, both in little-endian byte ordering:

  GZIP = GZIPHEADER(10B)  DEFLATE  CRC32(4B)  LENGTH(4B)

  This is the format of the .gz files, with the GZIPHEADER containing in the order the GZIP signature (\x1f\x8B), the compression method (\x08), a zero flag byte, a zero modification time on 4 bytes, an extra-flags byte that depends on the compression level, and the operating system set to the current system (\x00 = FAT filesystem, \x03 = Unix, etc.). This format is generated when the window parameter is set in the range from 9+16=25 up to 15+16=31. Note that there is a limit of 4GB to the maximum length of the uncompressed data; beyond this limit, only the modulo 2^32 of the actual length is stored in the LENGTH part. The corresponding functions that operate directly on strings are gzencode() and gzdecode(); the gzopen() function allows to read and write .gz files.

• DEFLATE (» RFC 1951): use window=-$W, with $W being a value not less than that used for compression. If unknown, set window=-15.

• ZLIB (» RFC 1950): use window=$W. The value of $W is available from the same ZLIB header, otherwise set window=15.

• GZIP (» RFC 1952): use window=$W+16. A GZIP header carries no window size, so set window=31 unless the value used for compression is known.

• ZLIB or GZIP: use window=$W+32 for automatic header detection, so that both the formats can be recognized and decompressed; window=15+32=47 is the safer choice.

Note:

Because compression level is the most commonly used parameter, it may be alternatively provided as a simple integer value (rather than an array element).

zlib.* compression filters are available if zlib support is enabled.

<?php
$params = array('level' => 6, 'window' => 15, 'memory' => 9);

$original_text = "This is a test.\nThis is only a test.\nThis is not an important string.\n";
echo "The original text is " . strlen($original_text) . " characters long.\n";

$fp = fopen('test.deflated', 'w');
stream_filter_append($fp, 'zlib.deflate', STREAM_FILTER_WRITE, $params);
fwrite($fp, $original_text);
fclose($fp);

echo "The compressed file is " . filesize('test.deflated') . " bytes long.\n";
echo "The original text was:\n";
/* Use readfile and zlib.inflate to decompress on the fly */
readfile('php://filter/zlib.inflate/resource=test.deflated');

/* Generates output:

The original text is 70 characters long.
The compressed file is 56 bytes long.
The original text was:
This is a test.
This is only a test.
This is not an important string.

 */
?>

<?php
$original_text = "This is a test.\nThis is only a test.\nThis is not an important string.\n";
echo "The original text is " . strlen($original_text) . " characters long.\n";

$fp = fopen('test.deflated', 'w');
/* Here "6" indicates compression level 6 */
stream_filter_append($fp, 'zlib.deflate', STREAM_FILTER_WRITE, 6);
fwrite($fp, $original_text);
fclose($fp);

echo "The compressed file is " . filesize('test.deflated') . " bytes long.\n";

/* Generates output:

The original text is 70 characters long.
The compressed file is 56 bytes long.

 */
?>

bzip2.compress and bzip2.decompress

bzip2.compress and bzip2.decompress work in the same manner as the zlib filters described above. The bzip2.compress filter accepts up to two parameters given as elements of an associative array: blocks is an integer value from 1 to 9 specifying the number of 100kbyte blocks of memory to allocate for workspace. work is also an integer value ranging from 0 to 250 indicating how much effort to expend using the normal compression method before falling back on a slower, but more reliable method. Tuning this parameter effects only compression speed. Neither size of compressed output nor memory usage are changed by this setting. A work factor of 0 instructs the bzip library to use an internal default. The bzip2.decompress filter only accepts one parameter, which can be passed as either an ordinary boolean value, or as the small element of an associative array. small, when set to a true value, instructs the bzip library to perform decompression in a minimal memory footprint at the cost of speed.

bzip2.* compression filters are available if bz2 support is enabled.

<?php
$param = array('blocks' => 9, 'work' => 0);

echo "The original file is " . filesize('LICENSE') . " bytes long.\n";

$fp = fopen('LICENSE.compressed', 'w');
stream_filter_append($fp, 'bzip2.compress', STREAM_FILTER_WRITE, $param);
fwrite($fp, file_get_contents('LICENSE'));
fclose($fp);

echo "The compressed file is " . filesize('LICENSE.compressed') . " bytes long.\n";

/* Generates output:

The original file is 3288 bytes long.
The compressed file is 1488 bytes long.

 */
?>