I have upgraded my Ubuntu system to latest one that is Ubuntu 13.10 (at the time of writing this).
I run on my machine Apache, PHP and MySQL as well as Drupal and drush. After upgrade to Ubuntu 13.10 I noticed that drush command doesn't work anymore - it did show following error:
PHP Fatal error: Call to undefined function json_encode() in /usr/share/php/drush/includes/output.inc on line 390
Reason for above error is that PHP json extension is not installed by default of Ubuntu 13.10. Please run this command: php -m | grep json to see that you do not have it.
To fix this problem I needed to install php-json extension: sudo apt-get php5-json
Now drush is working again and PHP json extension is available on my machine: php -m | grep json
That was quick fix once you know it. I hope you would find it helpful.