Recursively apply a function to all nodes of the tree at or below a certain node. The function is called for each node after it has been called for that node's children.

The callback parameter can be a function name as a string; this function will be called. If you want to use the function of an object, just use

array( &$obj, "function")

The third parameter is optional, it will be passed to the called function. You can have as many own parameters as you like.

The calling function needs to implement at least 2 parameters: The first is the tree object, the second the specific node.