PHP has noting to do with HTML, XML, DHTML, or any other formating language. PHP is for processing data. HTML and friends are only for displaying data.

PHP can't render anything but plain text - it is only by placing that text output within HTML that it creates anything other than plain text.

It is like numbers the sales department sends to the marketing department. It is up to the marketing dept. to choose how they place/show those numbers on their charts to the board of directors. I could be in a powerpoint presentation, with vinyl boards, or they could simply say them.

Just like it is up to you what you do with the PHP output. Should you place it in HTML or XML? OPML or DHTML? It really doesn't matter ;)