You want your PHP files (either a frontend web page or a purely backend script) to integrate with Postgres. You want .php files to log into a Postgres database to run SQL commands. How do you do this?
Install php5-pgsql or php7.0-pgsql. It depends on your version of PHP. Use this command to find out what version of PHP you have: php --version
Install this module: libapache2-mod-php. As root or with a "sudo " in front of this command, run:
apt-get -y install libapache2-mod-php
To install the php pgsql package on Ubuntu, you should run a command like one of these as root:
apt-get install php5-pgsql
apt-get install php7.0-pgsql