DB2 ($$$) has excellent XML integration, but you pay for it. This is what we use, where I work. You will have to (in any case) use the latest PHP if you intend to use PDO to interact with it.
I assume that you are looking for a database that can handle XML. From my understanding (rumors), Oracle and MS-SQL also process XML, but they are not polished.
In the open source world, I don't know what to recommend for tight XML integration.
-CF
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