Let's consider the following code :

class A{
	function test(int $x) : bool{
		return $x>0;
	}
}

class B extends A{
	function test($x){
		return $x>10;
	}
}

With PHP7.0+ default parameters, this code would trigger the following error :
Fatal error: Declaration of B::test($a) must be compatible with A::test(int $a): bool 

It could be useful if the Editor would flag this issuue as a warning or an error the definition of function test in class B.