Install memcache on ubuntu server by the 6 simple step. if apt-get not work in ubuntu server use aptitude instead of apt-get
Step 1

sudo apt-get update
or
sudo aptitude  update

Step 2

sudo apt-get install mysql-server php5-mysql php5 php5-memcache
or
sudo aptitude  install mysql-server php5-mysql php5 php5-memcache

Step3

sudo apt-get install memcached
or
sudo aptitude  install memcached

Step 4

sudo apt-get install php-pear
or
sudo aptitude  install php-pear

Step 5

sudo apt-get install build-essential
or
sudo aptitude  install build-essential

Step 6

sudo pecl install memcache

Step 7

echo "extension=memcache.so" | sudo tee /etc/php5/conf.d/memcache.ini

Restart The server and You will find out memcache in PHPinfo function
sudo /etc/init.d/apache2 restart