PHP Cookbook

18.25. Reading and Writing Compressed Files

Problem

You want to read or write compressed files.

Solution

Use PHP’s zlib extension to read or write gzip‘ed files. To read a compressed file:

$zh = gzopen('file.gz','r') or die("can't open: $php_errormsg");
while ($line = gzgets($zh,1024)) {
    // $line is the next line of uncompressed data, up to 1024 bytes 
}
gzclose($zh) or die("can't close: $php_errormsg");

Here’s how to write a compressed file:

$zh = gzopen('file.gz','w') or die("can't open: $php_errormsg");
if (-1 == gzwrite($zh,$s))   { die("can't write: $php_errormsg"); }
gzclose($zh)                or die("can't close: $php_errormsg");

Discussion

The zlib extension contains versions of many file-access functions, such as fopen( ), fread( ), and fwrite( ) (called gzopen( ) , gzread( ), gzwrite( ), etc.) that transparently compress data when writing and uncompress data when reading. The compression algorithm that zlib uses is compatible with the gzip and gunzip utilities.

For example, gzgets($zp,1024) works like fgets($fh,1024). It reads up to 1023 bytes, stopping earlier if it reaches EOF or a newline. For gzgets( ), this means 1023 uncompressed bytes.

However, gzseek( ) works differently than fseek( ). It only supports seeking a specified number of bytes from the beginning of the file stream (the SEEK_SET argument to fseek( )). Seeking forward (from the current position) is only supported in files opened for writing (the file is padded with a sequence of compressed zeroes). Seeking backwards ...