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I would like to send an email for each user registration.
What is the most proper place, where to put the code?
Your best bet is to write a custom user signup form, and tell the user app to use it instead of the default one. To do this, copy the file apps/user/handlers/signup.php into your own app (e.g., apps/mysite/handlers/signup.php) and make your edits there.
Also, in the file apps/user/conf/config.php find the [Custom Handlers] section and change the user/signup to point to your custom handler:
user/signup = mysite/signup
This will replace Elefant's own signup form with your custom one.
I followed the instructions but after the change the signup page is no longer available
Overwriting apps/user/handlers/signup.php without modifying custom handlers in apps/user/conf/config.php everything seems ok.
If you turn on the debug and display_errors settings in the global config, there's probably a PHP error resulting from the file moving into a new app.
That may help track the problem down and avoid having to modify the original source files (since that makes it harder to upgrade later).