xdiff is currently available through PECL http://pecl.php.net/package/xdiff.
If PEAR is available on your *nix-like
system you can use the pear installer to install the xdiff extension, by the
following command: pear -v install xdiff.
You can always download the tar.gz package and install xdiff by hand:
Example 1. xdiff install by hand
tar -xvf xdiff-xxx.tar
./configure && make && make install