Laravel Blade include: How to pass variable from child to parent?

For example, in parent.blade.php:

<?php $a = 20; ?>
{{$a}}
@include("child")
{{$a}}

and in child.blade.php:

{{$a}}
<?php $a = 10; ?>
{{$a}}

the ouput:

20
20
10
20

It's clear that the variable was passed from the parent to the child, but changing it inside the child doesn't affect the parent variable. In addition, if the variable wasn't declared in the parent before including the child, it will of course through an error if used after the inclusion.