hierarchical tree models

I am looking for a hierarchical tree model in MySQL that performs best with the following requirements:

• Multiple root nodes (and I mean a hell of a lot)
• Limited depth (4 levels max)
• Siblings sorted within structure
• Quick to use (SELECT)
• Doesn't need to be quick to manage (CREATE, UPDATE, DELETE)
• Has a good PHP API to manage an开发者_C百科d use

I use Nested Sets a lot but the multiple root nodes worry me in this instance. On the other hand Adjacency List may be too simple.

I've seen how Drupal handles its menus (Path Model???) but haven't been able to find a lot of information on the pro's and con's of this approach - and also if there is easily accessible API information. It looks interesting though.

Anyone have any other suggestions/references?