I posted here before about "this" being in get_defined_vars.

It turns out it's not always there but in certain cases it will inexplicably appear.

    php -r '
    class Test {
        public function a() {var_dump(array_keys(get_defined_vars()));$a = 123;}
        public function b() {var_dump(array_keys(get_defined_vars()));$this;}
    }
    $t = new Test();
    $t->a();
    $t->b();
    '

    array()
    array('this')

This does not happen in PHP 7.2 but will happen in PHP 5.6.