Filter in script how to implement?

Question

PHP

Good evening, please tell me how to check the script for forbidden words and empty fields.
The script works like clockwork, but how to make sure that the file is not created - if one or all fields are empty (not filled)? And if we allow to include the file include 'xx.php'; with a list of black words, so that if one of the words is present in the fields, then the form is also not created!