The answer is very trivial, but the documentation lacks a description, so I am posting it here (as well as adding it to the documentation of course):
private myArray = array();
There is a Mapping Type called "hash" which mappes a PHP array to a MongoDB one.
Of course you need to remember that an array in a MongoDB document cannot have any keys but only values. If you need keys, that would be a new document which gets embedded (see the docs on embedded documents).