PHP Static and Normal Functions Calling

Question

Can someone explain how these line of code are working

class DemoStatic {
  function normalFunction() {
    echo "This is the normal function";
  }
  static function staticFunction() {
    echo "This is the static function";
  }
}
$classObj = new DemoStatic();

DemoStatic::normalFunction(); vs. $classObj->normalFunction();

and

DemoStatic::staticFunction(); vs. $classObj->staticFunction();

And the output is as below

This is the normal function

This is the normal function

This is the static function

This is the static function

Now I really don't understand that how come a normal function is accessible via :: Operator and static function is called using the class object.

Can someone please explain?