post to custom php page?

Good question because there's an important gotcha to be noted. Views Bulk Operations supports two kinds of operations to be performed on selected nodes: those that are defined through hook_node_operations and those defined as actions. To perform your own logic, you can either implement hook_node_operations() in your own module, or create a new action. In either case, you will select that new operation in the Operations tab of the bulk view.

Now here comes the trick: if your logic needs to process several nodes at once, e.g. to compute an average, then your best best is probably using hook_node_operations. Why? Because this hook accepts an array of nodes to be processed, whereas actions only accept one node at a time (and thus VBO repeatedly calls the action with each selected node).

Thanks.