Install pdo for postgres Ubuntu
Try the packaged pecl version instead (the advantage of the packaged installs is that they're easier to upgrade):
apt-get install php5-dev
pecl install pdo
pecl install pdo_pgsql
or, if you just need a driver for PHP, but that it doesn't have to be the PDO one:
apt-get install php5-pgsql
Otherwise, that message most likely means you need to install a more recent libpq package. You can check which version you have by running:
dpkg -s libpq-dev
Pecl PDO package is now deprecated. By the way the debian package php5-pgsql now includes both the regular and the PDO driver, so just:
apt-get install php-pgsql
Apache also needs to be restarted before sites can use it:
sudo systemctl restart apache2