For simple web "app" like form and form processing where you are not using dynamic reload of page elements, you can drop JavaScript and go with PHP + HTML + CSS

For simple web "app" like form and form processing where you are not using dynamic reload of page elements, you can drop JavaScript and go with PHP + HTML + CSS. Use vanilla procedural PHP with sane splitting in functions / files. This stack is good enough for simple web apps, but also good for a little more complicated like for example task list etc. You can optionally sprinkle it with some vanilla JavaScript for dynamic parts, like showing disappearing error messages.