This method adds files and directories to an existing
archive. If the archive does not exist, it attempts to
create it. The files and directories listed are
added at the end of the archive, even if a file with
the same name is already archived.
Parameter
mixed $filelist - an array of filenames
and directory names, or a single string with names separated
by a single blank space.
For each directory added in the archive, the files and
sub-directories are also added.
Return value
boolean - Returns TRUE on success, FALSE on failure.