Description:
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(similar to https://bugs.php.net/bug.php?id=53036&edit=2, but more relevant issue summary - * see below)

Currently if a script wants to declare a class, and inherit from a missing parent class, a fatal error will occur.

This causes problems with class_exists() + autoload:
- If class C cannot be autoloaded, then class_exists('C') will simply return false.
- If class C can be autoloaded, but the parent class P can not, then class_exists('C') will include the file that defines C, which triggers a fatal error because P does not exist and cannot be autoloaded.

Typical scenario:
- Library LibC contains a class C with parent class P, where P is defined in another library LibP.
- Library LibC does NOT explicitly depend on LibP. Instead, the class C should only be used if LibP is installed in the project. We could say, LibP is an "optional dependency" of LibC.
- A discovery algorithm scans LibC for useful classes, and tries "class_exists('C')", or explicitly includes the file defining C -> fatal error, because P is missing.

Instead, such discovery algorithms nowadays need to use PHP parsing and stuff like Doctrine's StaticReflectionParser. https://github.com/doctrine/common/blob/master/lib/Doctrine/Common/Reflection/StaticReflectionParser.php

Solution:
- Instead of a fatal, simply behave as if the class C is not defined. So, class_exists('C') should return FALSE.
- Internally remember the "stub" definition of C.
- If class_exists('C') is called again, and autoloading for 'C' would be triggered again, instead trigger the autoload for 'P' and attempt to complete the stub definition of C.
- Provide a new language feature to test for "stub" class definitions.

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* Why a new issue, and not comment on https://bugs.php.net/bug.php?id=53036&edit=2 ?
The other issue paints this as a problem with class_exists() + autoload. But solution is to change the behavior of include(), independent of class_exists() + autoload. The description and code example in this issue focuses directly on the include(). I expect this to allow for a more focused discussion.

Test script:
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// Assuming class P does not exist and cannot be autoloaded either.
class C extends P {}

print class_exists('C') ? 'yes' : 'no';

Expected result:
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'no'
(because C has a non-existing base class)

Actual result:
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PHP Fatal error:  Class 'P' not found [..]

(triggered by the class definition, NOT by the class_exists())