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.
Access to undefined constants will now always result in an Error
exception being thrown.
This required quite a few test changes, because there were many
buggy tests that unintentionally used bareword fallback in combination
with error suppression.
Máté Kocsis