Unable to install elgg on 14.04.5, shows message "Elgg requires PHP 5.6.0 or above. This server is using version 5.5.9-1ubuntu4.21."

I'm newbie to elgg. I have followed instructions given on http://learn.elgg.org/en/stable/intro/install/ubuntu.html. But after all steps it shows message regarding older PHP version. I tried to install newest version still it shows that message.

Help me to install correct php version.