You can update PHP like shaun m says, or if you don't want to do that you can install zendopcache from PECL
Check for latest version here, this installs version 7.0.3.
Install "pecl" via php-pear
`sudo apt-get install -y php-pear`
Install build dependencies
sudo apt-get install -y build-essential php5-dev
Install Zend OpCache
sudo pecl install zendopcache-7.0.3
Find the location of opcache.so
sudo find / -name 'opcache.so'
Mine is
/usr/lib/php5/20100525/opcache.so
Now create opcache.ini
sudo vim /etc/php5/mods-available/opcache.ini
Add this info
zend_extension=/usr/lib/php5/20100525/opcache.so # File path from above
opcache.memory_consumption=128
opcache.interned_strings_buffer=8
opcache.max_accelerated_files=4000
opcache.revalidate_freq=60
opcache.fast_shutdown=1
opcache.enable_cli=1
Now create a symlink for the conf file
sudo ln -s /etc/php5/mods-available/opcache.ini /etc/php5/conf.d/20-opcache.ini
Restart services
sudo service php5-fpm restart
sudo service apache2 restart
Confirm install
php -v
It should give mention of
Zend Engine v2.4.0, Copyright (c) 1998-2013 Zend Technologies
with Zend OPcache v7.0.3, Copyright (c) 1999-2014, by Zend Technologies
Then (optionally) finish by cleaning up
sudo apt-get remove --purge build-essential php5-dev
Reference