The question as it is makes little sense. First of all, you need to understand how PHP or other server-side technologies work. Nothing is really "embedded" in PHP.
Any PHP file is actually some kind of template of what should be generated as a HTTP response in response to some HTTP request. So, this is some text file with fragment of PHP code in it; such fragments are escaped using the delimiters
. Such syntax is chosen to avoid interference with HTML syntax: the delimiters cannot be interpreted as HTML text elements (because "greater then" and "less then" brackets are not escaped with HTML entities), nor they are valid HTML tags. As the server side the PHP module first locates and interprets PHP fragments, it does not creates conflicts.
Actually, PHP code could generate a resource of any content type via generation of appropriate HTTP headers, but then the whole page should be the PHP code.
On HTTP request with URI pointing to some PHP page (which a HTTP server recognized by the file name or index file name, looking at the path and the content of some directory of the server's host file system), the PHP module loaded by the server interprets all the PHP code fragment and executes them; its generates some content to be inserted in the PHP file content in the place where the code fragment was. This is repeated sequentially for all such fragments.
If you understand this mechanism, you can generate any thinkable content. The content formatted according to the content of all fragments of PHP file is sent in the HTTP response to the client. No analysis of validity of the resulting content to be sent is not analyzed on the server side — the HTTP response is sent as is. On the client side, the content is loaded by the browser and interpreted according to the content type received in the HTTP header: rendered, JavaScipts are loaded, interpreted and executed, and so on.
Are you getting the idea? You should first separate what happens on server and client side. If you also know how HTML works, your questions about CSS are reduced to understanding on how it works in HTML. I hope you know it, or can read about it.
As to XAML, it is not directly related to PHP. XAML is used in WPF and Silverlight which also have nothing to do with PHP. The only thing about XAML can be some XAML library for reading and processing XAML files, which can be use as the content on client side only or processed on server side as any other data file. This is quite possible, because XAML is an extremely wide standard. A part of XAML can be used just a vector graphic format understood by browsers directly of via plug-ins. Alternatively, such graphics can be rendered as bitmap graphics purely on the server side and delivered to the client side through HTTP response as a bitmap content type (such as JPEG or PNG).
For further detail, please see: