I just finished this installation and ran into a couple of problems, so maybe someone else will benefit. If you haven’t read the PHP Tidy manual, you should.

First, install libtidy and libtidy-devel. If you don’t install libtidy-devel you will get an error later, during the configure step for the PHP library.

sudo yum install libtidy libtidy-devel

Second, download Tidy 2.0. Tidy 1.x is what I got via yum, but as the manual says, this is for PHP 4.x. If you have PHP 5.x, you want Tidy 2.0.

Get it via wget:
wget -c http://support.office-shadow.com/installer/tidy2.0.tar.gz

(Thank you to whoever runs this site.)

The rest of the instructions are as documented by Paul Maddox at http://ubuntuforums.org/showthread.php?t=195636:

# Unpack the source
tar xvzf tidy2.0.tar.gz

# Configure tidy for installed php5 API
cd tidy
phpize

# Configure & Compile the source
./configure
make clean    > /etc/php5/apache2/php.ini;

Thank you, Paul et al.

Note platform-specific location of the php.ini files. Mine is actually at /etc/php.ini on Fedora Core.

When this is done, you may need to restart your Apache httpd server. After you do, the output from phpinfo() should include a subsection labeled DOM with a few lines that say “enabled.”