You know, if you have to purge caches because you've updated a database table then there's something wrong with your code. If you explore Moodle's built-in functions, as I was trying to explain, you will find that it should work properly. Have a look inside the group/lib.php file. I think that will probably have most of the functions you need. If you treat Moodle as a framework, you will find it will do more of the lifting for you rather than just coming at it as a built from scratch PHP program. Unfortunately, I can't debug your code for you. Good luck. Mark
General developer forum
Removing & Adding a student from a group using php
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