ubuntu 14.04 amd64 LTS
Installed LAMP and phpmyadmin.
Cannot open phpmyadmin, error “json extension is missing.
Go to: http://packages.ubuntu.com/saucy/amd64/php5-json/download
To download one deb file and save to somewhere, my place is: /var/www/php5-json_1.3.1+dfsg-2_amd64.deb
To install the deb file: dpkg -i /var/www/php5-json_1.3.1+dfsg-2_amd64.deb
Restart apache2: sudo /etc/init.d/apache2 restart
Open phpmyadmin. It works!