Using php + sql and maybe a little javascript is generally an OK-ish method that has few strings attached and runs on anything. However there are a lot of modern frameworks that let you do a lot more stuff. There things like django, firebase, blazor, asp net or whatever but what FOSS equivalent to this “next gen” web development stuff should we be using?
“Web apps” is a broad category. Different tools have different tradeoffs, and a tool that’s great at building one kind of web app might be horrible for another kind. Without knowing more about a specific project, it’s pretty tough to make technology recommendations.