Should i choose HTML or PHP to build a web form ?

HTML is about the format of the document (the Document Object Model). It's about how the viewer will see the document (the client side). It can send stuff back to the server where your database is stored, but it really is about what the browser can translate into the user's screen

PHP is a server-side scripting language, a kind of pre-processor. Like a client-side script, it can modify the DOM, but, to the client, it looks like HTML.

So, you'll need both.

You will need both.

The browser renders HTML, so the underlying "code" of the form is HTML. However, on the server side there MUST be a code running that processes the form data and (if needed) modifies the generated form to do - for example- dynamic filling or validation feedback. PHP is one of the many languages that can do it. It could be JAVA, ASP, JAVASCRIPT (via node.js), PERL, C#, VB.net etc etc.

At a minimum one can make a plain HTML form that is being processed by the server-side code and a confirmation page.

Of course it is possible to produce all the form HTML markup in a server-side piece of code. Plain stupid, but possible.