Check if current function called statically or not

$static = !(isset($this) && get_class($this) == __CLASS__);
if ($static) {
  return self;
} else {
  return $this;
}

in class example:

class Foo {
   function bar() {
      $static = !(isset($this) && get_class($this) == __CLASS__);
if ($static) {
  return self;
} else {
  return $this;
}
   }
}

or simply create below function to test:

class A
{
    function foo()
    {
        if (isset($this)) {
            echo '$this is defined (';
            echo get_class($this);
            echo ")\n";
        } else {
            echo "\$this is not defined.\n";
        }
    }
}

How do I check in PHP that I'm in a static context (or not)?