PHP is a recursive acronym (the acronym of its name is part of the acronym for which its name stands) and means "PHP: Hypertext Preprocessor." All of that is a lot of lingo to say that PHP is a scripting language that generates HTML. Within a HTML document, it can be used to generate new HTML on the fly. For our purposes, it will serve as a CGI alternative to get data to our Python script.
Why could we not simply pass the form data directly to our program? Without CGI, there is no dialogue with the web server. There is no way of knowing whether the program actually worked without feedback. In the ensuing program, PHP serves as a relay for communication between the web page and the server.