Install PDO-driver for Postgresql

yum install php5-pgsql

pdo_pgsql is included in php5-pgsql.

You need to run for php 5.5:

yum install php55w-pgsql

For php 5.6 command is:

yum install php56w-pgsql

If you need another php extension, please read this article: PHP 5.6 on CentOS/RHEL 7.1 and 6.7 via Yum