Connect to the server first using SSH. If you are on the Linux or Mac it is a matter of seconds, on Windows you would be using Putty or web browser(Amazon AWS and GCP offer that).

Start with the customer – find out what they want and give it to them.

First of all get a sudo access:

sudo su

Update the Linux:

apt-get update && apt-get upgrade

Surely check if there is PHP already installed:

php -v

If your system says that it is already PHP installed and your PHP code is not working(for example contact form) then you might need to install only some additional packages.

For a list of the packages available:

apt-cache search --names-only ^php

Installing some popular modules:

apt-get install php-pear php7.0-dev php7.0-zip php7.0-curl php7.0-gd php7.0-mysql php7.0-mcrypt php7.0-xml

Yes, you guessed it right - you can choose your PHP version at this stage

Nginx 

has no native processing for PHP so you will have to install fpm:

sudo apt-get install php-fpm php-mysql

Once it is installed adjust the configuration by uncommenting cgi.fix_pathinfo and setting it to "0":

sudo nano /etc/php/7.0/fpm/php.ini

Restart both Nginx and fpm to apply changes:

sudo systemctl restart php7.0-fpm

Remember to update your Nginx config in sites-available

sudo nano /etc/nginx/sites-available/your-site.com

by adding:

    location ~ \.php$ {
        include snippets/fastcgi-php.conf;
        fastcgi_pass unix:/run/php/php7.0-fpm.sock;
    }
    location ~ /\.ht {
        deny all;
    }

Then restart Nginx:

service nginx restart

sudo nano /var/www/html/info.php

<?php
phpinfo();

References