You can customise the PHP.INI file on an individual domain level for your customers using the Plesk "php_admin_value" parameter inside "vhosts.conf & vhosts_ssl.conf" files, usually located under "/var/www/vhosts/domain.com/conf/".

For Example;

php_admin_value magic_quotes_gpc "1"
php_admin_value register_long_arrays "1"
php_admin_value display_errors "1"