Description:
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The current invariative type interferes with writing idiomatic inheritance, which is implemented in many other languages. 

Now us have to completely remove the type of the return value and type and remove functions type hintings arguments in the parent and childs class, which helps to solve this. But in this case, in business logic now you need to manually implement type checking, since a secure application should work with the correct type. 

At the advent of PHP 7 with the possibility of strict typing, I hoped that the types will finally become variative. However, this did not happen. 

RFC by return types (https://wiki.php.net/rfc/return_types) in section "Variance and Signature Validation" has next: 
> This RFC originally proposed covariant return types but was changed to invariant because of a few issues. It is possible to add covariant return types at some point in the future. 

I think it's time to move towards idiomatic inheritance, since it will be correct if PHP to check the types, not the programmer.

Test script:
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Test script: https://gist.github.com/hack3p/78eeca8f50d3297bf25768a9c72cafeb

Expected result:
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Previous test script should be work

Actual result:
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example 1: return type
PHP Fatal error:  Declaration of B::foo(): B must be compatible with A::foo(): A in ...

example 2: arguments type hinting
PHP Warning:  Declaration of B::foo(B $bar) should be compatible with A::foo(A $bar) in ...