Uso de __CLASS__ vs ::class

Code

class A {

    public function getClassName(){
        return __CLASS__;
    }

    public function getRealClassName() {
        return static::class;
    }
}

class B extends A {}

$a = new A;
$b = new B;

echo $a->getClassName();      // A
echo $a->getRealClassName();  // A
echo $b->getClassName();      // A
echo $b->getRealClassName();  // B

Output

AAAB