Hi. First of all, all forms are actually HTML. What you probably have in PHP is the code that receives the form data. Maybe the form is dinamically created by PHP too, but that's very unlikely, and anyway, the result is also HTML. So it will be simple to convert the form into HTML.
Though, if you need to process the data and do something with it (like sending an email, inserting data into a database, etc) you will still need PHP or some other scripting language, but it won't be necessary to have that PHP code embedded into a page. The HTML form could just point to the PHP that processes it and then the user will be redirected to another HTML page.
Anyway, there are many ways to solve this and none of them is really complicated.
I can do this for you the best possible way. Greetings. Ruben Gutierrez