Technically, the proper way would be creating a PHP Wappler extension so you can require/include a PHP script. I'm imagining a custom Step where you can fill in the path of the PHP script you want to include
The less correct way would be opening the .php file in a code editor and manually include the PHP script there I suggest the first option instead
P.S.: I don't use Wappler with PHP, so I don't know if my advice is correct
You can maybe take a look at a simple PHP Wappler extension to see how it's done:
I suggest the first option instead