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
gunzip xdiff-xxx.tgz tar -xvf xdiff-xxx.tar cd xdiff-xxx phpize ./configure && make && make install