Sometimes pecl install tidy may not work. Or even yum install php-tidy

Then building from sources might help:

# Download sources
svn co http://svn.php.net/repository/php/php-src/branches/PHP_5_3/ext/tidy/

# Configure tidy for installed php5 API
cd tidy
phpize

# Configure & Compile the source
./configure
make clean <-- Without this the compile builds a bad module for some reason
make
make install