RFC: https://wiki.php.net/rfc/tostring_exceptions
And convert some object to string conversion related recoverable
fatal errors into Error exceptions.
Improve exception safety of internal code performing string
conversions.
E_RECOVERABLE errors are reported as "Catchable fatal error". This is
misleading, because they actually can't be caught via try-catch statements.
Therefore we change the wording to "Recoverable fatal error" as suggested by
Nikita.
