# yum install libmcrypt libmcrypt-devel

# cd /usr/local/src/php-5.2.17/ext/mcrypt
# /usr/local/php-5.2.17/bin/phpize
# ./configure --with-php-config=/usr/local/php-5.2.17/bin/php-config
# make
# make test
# make install

# cp /usr/local/php-5.2.17/lib/php/extensions/no-debug-zts-20060613/mcrypt.so /usr/local/php-5.2.17/lib/php/extensions/

# vim /usr/local/php-5.2.17/etc/php.ini
extension_dir = "/usr/local/php-5.2.17/lib/php/extensions/"
extension=mcrypt.so