Source: How to install OCI8 on Ubuntu 16.04 and PHP 7.1

Download the Oracle Instant Client and SDK from Oracle website. (Need to login in Oracle page)

http://www.oracle.com/technetwork/topics/linuxx86-64soft-092277.html

Files: instantclient-basic-linux.x64-18.3.0.0.0dbru.zip and instantclient-sdk-linux.x64-18.3.0.0.0dbru.zip.

Create a new folder to store Oracle Instant Client zip files on your server.

Upload the Instant Clients files inside this folder.

mkdir /opt/oracle

Now we need to extract the files.

unzip instantclient-basic-linux.x64-18.3.0.0.0dbru.zip -d /opt/oracle
unzip instantclient-sdk-linux.x64-18.3.0.0.0dbru.zip -d /opt/oracle

Add the folder to our ldconfig.

echo /opt/oracle/instantclient_18_3 > /etc/ld.so.conf.d/oracle-instantclient

Update the Dynamic Linker Run-Time Bindings

ldconfig

Done. Now we can proceed to the next part.

To install the OCI8 extension, we need to install some additional package on our server.

Run these command:

apt install php-dev php-pear build-essential libaio1

Once installed, we need to get the OCI8 file.

pecl install oci8

When you are prompted for the Instant Client location, enter the following:

instantclient,/opt/oracle/instantclient_18_3

We need to tell PHP to load the OCI8 extension.

echo "extension = oci8.so" >> /etc/php/7.2/fpm/php.ini
echo "extension = oci8.so" >> /etc/php/7.2/cli/php.ini

Add the following lines to /etc/apache2/envvars

export LD_LIBRARY_PATH=/opt/oracle/instantclient_18_3
export ORACLE_HOME=/opt/oracle/instantclient_18_3

Check if the extension is enabled.

php -m | grep 'oci8'

If returns oci8, its works!

Restart the PHP-FPM

service php7.2-fpm restart

Now you can connect to Oracle DBMS from your PHP applications.