Description:
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This occurs with all versions of PHP it seems. I've tested with the current snap:

PHP 7.1.0-dev (cli) (built: Mar  6 2016 06:13:27) ( NTS )
Copyright (c) 1997-2016 The PHP Group
Zend Engine v3.1.0-dev, Copyright (c) 1998-2016 Zend Technologies

To reproduce, store 2 class instances statically. Even if class (Foo) stores a ref to class (Bar), if (Bar) is initialized first it is destructed first, allowing (Foo) to access an object whose destructor has already been called.

It seems that objects stored in static (global) refs are destroyed based on those globals only rather than potential local refs.

I would expect the destructor for (Foo) to fire first because Foo has a ref to (Bar)

Test script:
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https://3v4l.org/Ifgn9

Expected result:
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Bar::__construct
Foo::__construct
Foo::__destruct
Bar::func
Bar::__destruct

Actual result:
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Bar::__construct
Foo::__construct
Bar::__destruct
Foo::__destruct
Bar::func