How can you set a custom error handler in PHP?

You can use `set_error_handler()` to define a custom function that will be called whenever a PHP error occurs. This function receives the error number, message, file, and line number as arguments. This allows you to log errors to a file, send email notifications, or display custom error messages instead of the default PHP error output. Remember to restore the original error handler with `restore_error_handler()` when finished.