CiviCRM Initalization Error: DB Error: extension not found
This turned out to be the issue with PEAR trying to load DB (mysql) extension. PHP mysql extension is removed from PHP7. 

For those who landed here with same errors, when trying to execute commands with civix or drush which uses command-line php (php-cli), if your OS has installed both PHP5 and PHP7, the default php command available in /usr/bin/php refers to PHP7 while PHP5 executable will be available as /usr/bin/php5

One needs to update /usr/bin/php to /usr/bin/php7 and direct /usr/bin/php5 towards /usr/bin/php. Maybe using update-alternatives would be best suited.