XML doubt

I don't know how to work with XML... I know that it's a good standard to pass information between computer or applications, but I don't know how to separe my data from my presentation on my PHP projects. I always use OOP, I organize all the data in databases and clases, but I always have to put some code on my presentation files, like index.php, viewstock.php, and it will be great if I can organize my projects so the presentation can be handled by any designersm, at this time I always have to make the changes on the presentation files because I am the one who know about PHP. Can you give some example about how to separate the data from the presentation using XML and databases? Or can you tell me about a project where I can see it? Thank you.