Can one mix different programing languages?

Answer 1

Mix as in use together in the same file, no. Mix as in use together, yes.

Answer 2

Maybe not the way you are thinking. For instance on a web project you might interact with CSS, HTML, JS, PHP, MySQL. You can also call functions from imported DLLs that are written in different languages.

Answer 3

Well the closest you could get would be using php ti generate js on the fly... but not really because php will get parsed to html and javascript and not be php any more when it hits the browser.