Action Hooks

From WHMCS Documentation

Action hooks allow you to write your own PHP code that will run when specific actions occur in WHMCS. There is a function for each action that supports hooks.

To view the available action hooks, open the file includes/actionhooks.php (The variables avaible in a specific function are listed in the comments for that function).

Invalid code here will will cause WHMCS to fail so ensure your code is valid.