yum install memcached php-pecl-memcache
pecl install memcache

if pecl fail you can install from source:
wget http://pecl.php.net/get/memcache
tar xvfz memcache-*
cd memcache*
phpize
./configure
make && make install
echo extension=memcache.so >> /etc/php.ini

service httpd restart
service memcached start
or maybe run manually:
memcached -d -m 512 -l 127.0.0.1 -p 11211 -u nobody