Location of php.ini file in plesk server is inside etc folder. you can login using your details (provided by hosting provider) in to ssh login prompt after you can type below commant to edit your php.in file.
# vim /etc/php.ini
you can type “i” key to make this file editale, you can use “esc” command to escape from editable mode .To save you can use “wq:”.
you can always use phpinfo(); to check the exact location of php in server.
create a php file e.g info.php with below conntent
echo phpinfo();
save this and upload in your root folder
check “domain.com/info.php” it will show you exact location of “php.ini ” files.Also it will show all the details about your php version.