Functions.php scope

I wrote a form for my template and have a php file that processes the form. I would like to include the functions.php file in my process form php file so that I can call various functions from it (such as creating a new draft post with the form data) But when i include functions.php in the processForm.php, it doesn’t recognize all the wordpress functions like add_filter etc.

Basically, I don’t understand how new php files in a theme can understand wordpress directives.

Thanks!