gzip是GNU zip的缩写，它是一个GNU自由软件的文件压缩程序，也经常用来表示gzip这种文件格式。
HTTP／1.1协议允许客户端可以选择要求从服务器下载压缩内容，这个标准本身定义了两种压缩方法：“gzip”（内容用gzip数据流进行封装）以及“deflate”（内容是原始格式、没有数据头的DEFLATE数据流）。许多HTTP客户端库以及绝大多数当今的浏览器都支持这两种格式。在用户浏览器发送请求的HTTP头中, 带有“Accept-Encoding: gzip, deflate”参数则表明支持gzip和deflate两种压缩算法。
gzip命令的常用选项
-c，-stdout 将解压缩的内容输出到标准输出，原文件保持不变
-d，-decompress 解压缩
-f，-force 强制覆盖旧文件
-l，-list 列出压缩包内储存的原始文件的信息(如，解压后的名字、压缩率等)
-n，-no-name 压缩时不保存原始文件的文件名和时间戳，解压缩时不恢复原始文件的文件名和时间戳(此时，解出来的文件，其文件名为压缩包的文件名)
-N，-name 压缩时保存原始文件的文件名和时间戳，解压缩时恢复原始文件的文件名和时间戳
-q，-quiet 抑制所有警告信息
-r，-recursive 递归
-t，-test 测试压缩文件完整性
-v，-verbose 冗余模式(即显示每一步的执行内容)
-1、-2、…、-9 压缩率依次增大，速度依次减慢，默认为-6
使用Gzip压缩时的一些注意事项：
Gzip不仅可以对HTML、PHP等文件进行压缩，对CSS，JS等文件同样也可以；Gzip压缩会增加服务器的负载；不要使用Gzip对非文本（图片，视频，音频等）文件进行压缩；有选择性的使用Gzip压缩（对文本代码较多的页面进行压缩）；并不是最高的压缩率，压缩的文件就最小；高压缩率同时意味着高服务器负载；
建议把压缩率设为5~8；IIS，Apache1.3，Apache2.0+启用Gizp，配置方法是不同的；未开启Gizp压缩模块的虚拟主机（如Godaddy），只要加载了Zlib库，同样可以实现；
如果你的服务器不支持HTTP压缩，但支持PHP，那么下面的代码就可能对你很有用，下面的代码会自动压缩文件并给浏览器发送缓存设置。
<?php
ob_start();
/*
* The mkdir function does not support the recursive
* parameter in the version of PHP run by Yahoo! Web
* Hosting. This function simulates it.
*/
function mkdir_r( $dir_name, $rights=0777 ) {
$dirs = explode( "/", $dir_name );
$dir = "";
foreach ( $dirs as $part ) {
$dir .= $part . "/";
if ( !is_dir( $dir ) && strlen( $dir ) > 0 )
mkdir( $dir, $rights );
}
}
/*
* List of known content types based on file extension.
* Note: These must be built-in somewhere...
*/
$known_content_types = array(
"htm" => "text/html",
"html" => "text/html",
"js" => "text/javascript",
"css" => "text/css",
"xml" => "text/xml",
"gif" => "image/gif",
"jpg" => "image/jpeg",
"jpeg" => "image/jpeg",
"png" => "image/png",
"txt" => "text/plain"
);
/*
* Get the path of the target file.
*/
if ( !isset( $_GET["uri"] ) ) {
header( "HTTP/1.1 400 Bad Request" );
echo( "<html><body><h1>HTTP 400 - Bad Request</h1></body></html>" );
exit;
}
/*
* Verify the existence of the target file.
* Return HTTP 404 if needed.
*/
if (($src_uri = realpath( $_GET["uri"] )) === false) {
/* The file does not exist */
header( "HTTP/1.1 404 Not Found" );
echo( "<html><body><h1>HTTP 404 - Not Found</h1></body></html>" );
exit;
}
/*
* Verify the requested file is under the doc root for security reasons.
*/
$doc_root = realpath( "." );
if (strpos($src_uri, $doc_root) !== 0) {
header( "HTTP/1.1 403 Forbidden" );
echo( "<html><body><h1>HTTP 403 - Forbidden</h1></body></html>" );
exit;
}
/*
* Set the HTTP response headers that will
* tell the client to cache the resource.
*/
$file_last_modified = filemtime( $src_uri );
header( "Last-Modified: " . date( "r", $file_last_modified ) );
$max_age = 300 * 24 * 60 * 60; // 300 days
$expires = $file_last_modified + $max_age;
header( "Expires: " . date( "r", $expires ) );
$etag = dechex( $file_last_modified );
header( "ETag: " . $etag );
$cache_control = "must-revalidate, proxy-revalidate, max-age=" . $max_age . ", s-maxage=" . $max_age;
header( "Cache-Control: " . $cache_control );
/*
* Check if the client should use the cached version.
* Return HTTP 304 if needed.
*/
if ( function_exists( "http_match_etag" ) && function_exists( "http_match_modified" ) ) {
if ( http_match_etag( $etag ) || http_match_modified( $file_last_modified ) ) {
header( "HTTP/1.1 304 Not Modified" );
exit;
}
} else {
error_log( "The HTTP extensions to PHP does not seem to be installed..." );
}
/*
* Extract the directory, file name and file
* extension from the "uri" parameter.
*/
$uri_dir = "";
$file_name = "";
$content_type = "";
$uri_parts = explode( "/", $src_uri );
for ( $i=0 ; $i<count( $uri_parts ) - 1 ; $i++ )
$uri_dir .= $uri_parts[$i] . "/";
$file_name = end( $uri_parts );
$file_parts = explode( ".", $file_name );
if ( count( $file_parts ) > 1 ) {
$file_extension = end( $file_parts );
$content_type = $known_content_types[$file_extension];
}
/*
* Get the target file.
* If the browser accepts gzip encoding, the target file
* will be the gzipped version of the requested file.
*/
$dst_uri = $src_uri;
$compress = true;
/*
* Let's compress only text files...
*/
$compress = $compress && ( strpos( $content_type, "text" ) !== false );
/*
* Finally, see if the client sent us the correct Accept-encoding: header value...
*/
$compress = $compress && ( strpos( $_SERVER["HTTP_ACCEPT_ENCODING"], "gzip" ) !== false );
if ( $compress ) {
$gz_uri = "tmp/gzip/" . $src_uri . ".gz";
if ( file_exists( $gz_uri ) ) {
$src_last_modified = filemtime( $src_uri );
$dst_last_modified = filemtime( $gz_uri );
// The gzip version of the file exists, but it is older
// than the source file. We need to recreate it...
if ( $src_last_modified > $dst_last_modified )
unlink( $gz_uri );
}
if ( !file_exists( $gz_uri ) ) {
if ( !file_exists( "tmp/gzip/" . $uri_dir ) )
mkdir_r( "tmp/gzip/" . $uri_dir );
$error = false;
if ( $fp_out = gzopen( $gz_uri, "wb" ) ) {
if ( $fp_in = fopen( $src_uri, "rb" ) ) {
while( !feof( $fp_in ) )
gzwrite( $fp_out, fread( $fp_in, 1024*512 ) );
fclose( $fp_in );
} else {
$error = true;
}
gzclose( $fp_out );
} else {
$error = true;
}
if ( !$error ) {
$dst_uri = $gz_uri;
header( "Content-Encoding: gzip" );
}
} else {
$dst_uri = $gz_uri;
header( "Content-Encoding: gzip" );
}
}
/*
* Output the target file and set the appropriate HTTP headers.
*/
if ( $content_type )
header( "Content-Type: " . $content_type );
header( "Content-Length: " . filesize( $dst_uri ) );
readfile( $dst_uri );
ob_end_flush();
?> 上面的PHP文件下载：http://www.box.net/shared/e5vvdtg1no