# Introduction

This module enables you to transparently read and write gzip (.gz) compressed files, through versions of most of the filesystem functions which work with gzip-compressed files (and uncompressed files, too, but not with sockets).

> Note:

> Version 4.0.4 introduced a fopen-wrapper for .gz-files, so that you can use a special zlib: URL to access compressed files transparently using the normal f*() file access functions if you prefix the filename or path with zlib: when calling fopen(). This feature requires a C runtime library that provides the `fopencookie()` function. Up to now the GNU libc seems to be the only library that provides this feature.

> In PHP 4.3.0, zlib: has been changed to compress.zlib:// to prevent ambiguities with filenames containing '`:`' characters. The `fopencookie()` function is no longer required. More information is available in the section about zlib://.