I'm trying to build a custom form without any extension. And to do this I need to add a PHP form processor file in template
Is it secure to add and call form processor from the Joomla! Template?
This really depends on what you have in the
process.php file, and whether you are using proper filtering for user input or not.
Having said that, maintaining such a form is not fun, especially if the one maintaining it is not you. I suggest you use a real form builder such as RS Forms, which allows you to do almost anything form-building and form-processing wise.