(PHP 4 >= 4.0.1, PHP 5, PHP 7, PHP 8)

gzcompressCompress a string


gzcompress(string $data, int $level = -1, int $encoding = ZLIB_ENCODING_DEFLATE): string|false

This function compresses the given string using the ZLIB data format.

For details on the ZLIB compression algorithm see the document "» ZLIB Compressed Data Format Specification version 3.3" (RFC 1950).


This is not the same as gzip compression, which includes some header data. See gzencode() for gzip compression.



The data to compress.


The level of compression. Can be given as 0 for no compression up to 9 for maximum compression.

If -1 is used, the default compression of the zlib library is used which is 6.


One of ZLIB_ENCODING_* constants.


The compressed string or false if an error occurred.


示例 #1 gzcompress() example

= gzcompress('Compress me', 9);


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User Contributed Notes 2 notes

23 years ago
No, it doesn't return gzip compressed data -- specifically, the CRC is messed up. However, after massaging the output a lot, I have come up with a solution. I also commented it a lot, pointing out odd things.

// Start the output buffer

// Output stuff here...

// Get the contents of the output buffer
$contents = ob_get_contents();

// Tell the browser that they are going to get gzip data
// Of course, you already checked if they support gzip or x-gzip
// and if they support x-gzip, you'd change the header to say
// x-gzip instead, right?
header("Content-Encoding: gzip");

// Display the header of the gzip file
// Thanks ck@medienkombinat.de!
// Only display this once
echo "\x1f\x8b\x08\x00\x00\x00\x00\x00";

// Figure out the size and CRC of the original for later
$Size = strlen($contents);
$Crc = crc32($contents);

// Compress the data
$contents = gzcompress($contents, 9);

// We can't just output it here, since the CRC is messed up.
// If I try to "echo $contents" at this point, the compressed
// data is sent, but not completely. There are four bytes at
// the end that are a CRC. Three are sent. The last one is
// left in limbo. Also, if we "echo $contents", then the next
// byte we echo will not be sent to the client. I am not sure
// if this is a bug in 4.0.2 or not, but the best way to avoid
// this is to put the correct CRC at the end of the compressed
// data. (The one generated by gzcompress looks WAY wrong.)
// This will stop Opera from crashing, gunzip will work, and
// other browsers won't keep loading indefinately.
// Strip off the old CRC (it's there, but it won't be displayed
// all the way -- very odd)
$contents = substr($contents, 0, strlen($contents) - 4);

// Show only the compressed data
echo $contents;

// Output the CRC, then the size of the original

// Done. You can append further data by gzcompressing
// another string and reworking the CRC and Size stuff for
// it too. Repeat until done.

function gzip_PrintFourChars($Val)
for (
$i = 0; $i < 4; $i ++)
chr($Val % 256);
$Val = floor($Val / 256);
detain at interserver dot net
6 years ago
gzipped strings include header/metadata you can use to determine if a string is gzipped or not , but since gzcompress does not include that I found myself needing a way to determine if a string was compressed or not. After some research (and then improvements) i came up with this:

* determines if a string is a gzipped string supporting strings
* encoded with either gzencode or gzcompress
* @param string $string the string to check for compression
* @return bool whether or not the string was compmressed
function is_gzipped($string) {
return mb_strpos($string, "\x1f\x8b\x08", 'US-ASCII') === 0 && @gzuncompress($string) !== FALSE;
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