Usually this error occurs due to the outdated version of the pear very much which can no longer talk to the pear server and loads the HTML page instead of the real archive.
To fix this, you need to update the pear manually.
wget http://pear.php.net/go-pear.phar php go-pear.phar
And follow the instructions to install the system .
Besides,
unable to unpack
it may mean that you cannot extract gzip archives or that the drive /tmp/ full or something is connected. This is not all that is possible, but in such cases:
pear install -Z pear
will load .tar, not .tgz