first of all check the fragments you have in /etc/e-smith/templates/opt/remi/php56/root/etc/php.ini/ for your variable.
If you find in a fragment (say XYfile), copy it in /etc/e-smith/templates-custom/opt/remi/php56/root/etc/php.ini/, edit it and expand the template
if you don't find your key, just add a fragment into /etc/e-smith/templates-custom/opt/remi/php56/root/etc/php.ini/ dir, naming as you prefer.. the name should start with 2 numbers and the numbers you choose will be used to order the fragments in template expansion procedure.. IOW, a 12file content will appear in the first lines of the expanded file, while a 99file2 will be at the end.
Since the order of the variables in php.ini is not important, choose the numbers to add your key to the end of file (my personal taste)
HTH