looking for a db abstraction/substitute that actually works

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i am looking for a form of data storage that will answer a few requirements. i realize these requirements are non-standard, and for now i'm using activerecord and ORM solutions like everyone else, but this is my "holy grail" - if you know of anything like this, i would be eternally grateful:[*]pure PHP[*]multiple repositories, preferably file based for portability, where i can instantiate by telling it "use repository [X]" - i don't want to pre-create repository [X], if i reference it, it exists.[*]zero database configuration - i don't want to create tables or export SQL dumps, if it's referenced in my code, it needs to be in the database, auto-created without any fuss, my code is my schema[*]hierarchical, not relational, ideal structure would be just a freeform, schema-less XML, but since XML performs horribly with large trees, it can't simply be an XML file.i have experimented with flat XML storage (with xpath and xquery) but it gags on a mid-sized repository, and cripples the application.i have also experimented with key=>value pairs dropped into a SQLite database with a single generic table, but that gags even faster, and re-forming even the simplest record from key=>value pairs is a performance decimator.finally, i experimented with lucene as implemented in the zend framework, which was pretty close to ideal, apart from the no-update part.any ideas, anyone?