PHP 8.4 deprecates the use of `trigger_errror()` with `E_USER_ERROR` as the error level, as there are a number of gotchas to this way of creating a `Fatal Error` (`finally` blocks not executing, destructors not executing). The recommended replacements are either to use exceptions or to do a hard `exit`.
WP has its own `wp_trigger_error()` function, which under the hood calls `trigger_error()`. If passed `E_USER_ERROR` as the `$error_level`, this will hit the PHP 8.4 deprecation.
Now, there were basically three options:
* Silence the deprecation until PHP 9.0 and delay properly solving this until then. This would lead to an awkward solution, as prior to PHP 8.0, error silencing would apply to all errors, while, as of PHP 8.0, it will no longer apply to fatal errors. It also would only buy us some time and wouldn't actually solve anything.
* Use `exit($status)` when `wp_trigger_error()` is called with `E_USER_ERROR`. This would make the code untestable and would disable handling of these errors via custom error handlers, which makes this an undesirable solution.
* Throw an exception when `wp_trigger_error()` is called with `E_USER_ERROR`. This makes for the most elegant solution with the least BC-breaking impact, though it does open it up to the error potential being "caught" via a `try-catch`. That's not actually a bad thing and is likely to only happen for those errors which can be worked around, in which case, it's a bonus that that's now possible.
The third option is implemented which:
* Introduces a new `WP_Exception` class.
* Starts using `WP_Exception` in the `wp_trigger_error()` function when the `$error_level` is set to `E_USER_ERROR`.
This change is covered by pre-existing tests, which have been updated to expect the exception instead of a PHP error.
Why not use `WP_Error`?
Well, for one, this would lead to completely different behaviour (BC).
As `WP_Error` doesn't extend `Exception`, the program would not be stopped, but would continue running, which would be a much bigger breaking change and carries security risks. `WP_Error` also doesn't natively trigger displaying/logging of the error message, so in that case, it would still need an `exit` with the error message, bringing us back to point 2 above.
Introducing `WP_Exception` provides (essentially) the same behaviour in that it retains the fatal error and error message displaying/logging behaviors. It also introduces a base Exception class, from which future exception classes can extend.
References:
* https://wiki.php.net/rfc/deprecations_php_8_4#deprecate_passing_e_user_error_to_trigger_error
* https://www.php.net/manual/en/migration80.incompatible.php
Follow-up to [56530].
Props jrf, hellofromTonya.
See #62061.
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