The constructor declares a new Archive_Tar object, identifying it by the name of the tar file.
string $tarname - the name of the tar archive to work with
mixed $compress - if TRUE, indicates that the archive is compressed (using gzip). if NULL the archive is not compressed, if 'gz' or 'bz2', indicates that the archive is compressed with gzip or bz2. For compatibility reason the boolean value TRUE means 'gz'.