My site is completely xhtml/css. As photoalbum, I use Zenphoto. I wanted to implement the random foto funcion on my indexpage, but it's xhtml and the code for the random function is php.
How can I implement that function on my page?

You can mix xhtml with php code.... Take a look at zenphoto's themes and the theming tutorial. If your pages have the .html suffix you probably have either to reconfigure your server to accpet php code on .html pages or change the suffix to .php.