build a personal php cms- best practices

I would appreciate your opinion on the best methodologically or best practices to make a back office of a website. Factors to be taken into account are the site performance (speed of loading) and security.

The first hypothesis is only using PHP and MySQL, where the data come from frontoffice database.

The second hypothesis is using PHP MySQL and XML. The contents come from XML file that receives and writes data to and from the Data Base

In the second case I would have to use in the front office some library to parse the data from the XML to the PHP, right?

What do you consider the advantage of using one of these approaches instead the other?

The building of good CMS is not trivial, and for doing that in a good way, I would first try to understand some existing PHP CMS-es, frameworks and libraries. Good point to start is "list of web frameworks" on wikipedia.

I suggest that with high probability, the CMS you need - is already there. Designed, implemented, tested and bug-free :)

If you really need to build your own CMS, consider also storing as flat files (with cache or something), JSON, BJSON - because XML is not always a "best" option (and in the PHP-world XML is one of the slowest option probably). Also look at some already-done ADO implementations like Doctrine.