Materials to read on command bus, commands and events in Symfony:
https://www.uvd.co.uk/blog/using-the-command-bus-in-symfony/
http://simplebus.github.io/SymfonyBridge/doc/getting_started.html
Series of blogposts from Matthias Noback (the creator of simple-bus/SymfonyBridge bundle):
https://matthiasnoback.nl/tags/SimpleBus/
Nice presentation showing example of usage of simple bus - example includes a case of User Registration and email notification:
https://www.slideshare.net/JornOomen/crafting-beautiful-software