> the real reason is that non-silent autoloaders (for instance the Doctrine one) are breaking class_exists by throwing an error when trying to autoload a non-existent class.

That is somehow ironic, isn't? ...

> An autoloader should always be silent to preserve class_exists, but unfortunately, some
> autoloaders (ZF1 and Doctrine for instance) made the opposite choice

So any chance to get Doctrine somehow de-facto-standard compliant? And regarding ZF1 I have no idea .. Actually the whole PR seems pointless as long as there are autoloaders, that don't work as expected.