Description:
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I believe class inheritance diagrams would be beneficial in helping people to understand relations of some of the internal classes and interfaces and also would promote better use of exceptions through understanding.

Currently some classes like BadFunctionCallException don't list their descendants at all.

SPL documentation that was originally in ~helly/php/ext/spl/ had a great way of explaining class inheritance through clickable diagrams. I believe those were made with dot and doxygen.

Found an SPL documentation example from
http://fossies.org/dox/php-5.2.17/classException.html

Feature could be used at least in:
 DateTimeInterface -> DateTime, DateTimeImmutable,
 Exceptions and SPL Exceptions...