Where is the php.ini file on a Linux/CentOS PC?

On most installs you can find it here:

/etc/php.ini

php -i |grep 'Configuration File'

In your terminal/console (only Linux, in windows you need Putty)

ssh user@ip
php -i | grep "Loaded Configuration File"

And it will show you something like this Loaded Configuration File => /etc/php.ini.

ALTERNATIVE METHOD

You can make a php file on your website, which run: <?php phpinfo(); ?>, and you can see the php.ini location on the line with: "Loaded Configuration File".

Update This command gives the path right away

cli_php_ini=php -i | grep /.+/php.ini -oE  #ref. https://stackoverflow.com/a/15763333/248616
    php_ini="${cli_php_ini/cli/apache2}"   #replace cli by apache2 ref. https://stackoverflow.com/a/13210909/248616

You can find the path to php.ini in the output of phpinfo(). See under "Loaded Configuration File".