Here it is a ready-to-use example on how to extend Advanced Workflows leveraging UI configurable custom PHP actions. It is based on the original example from SugarCON last year.
I've made it so that it is module aware (the developer can define module specific methods) and the method to call can be configured from the UI (only module specific methods will appear on the workflow).
The code is here: https://github.com/esimonetti/SugarAdvancedWorkflowCustomPHPMethods
It will be extremely beneficial in situations where complex actions are required (eg: multiple lookups and records save at once), and UI action configuration/selection at the workflow level is required. The admin users with this type of customisation can have control on when and how the complex actions executed on the PHP side are triggered and have the flexibility to change the conditions that trigger them through the workflow UI.
The custom actions can be sent to the background and executed from the PHP side asynchronously, leveraging further our product's job queue to improve UI responsiveness for end users.
This should not be used as the silver bullet for everything, but it is a solid example to start with.
Feel free to have a look at the repository for more info!