The following (https://3v4l.org/A2Tp6) is ok, it simply returns false:
$x = 1; $x instanceof \stdClass; The following (https://3v4l.org/IdSBu) gives a fatal error: 1 instanceof \stdclass; t think this behavior is inconsistent, and we should consider changing it. There are two options, but only one is BC. - Let 1 instanceof \stdClass return false, instead of crashing. -> seems BC - Let $x instanceof \stdClass crash, if $x is not an object. -> BC break. So it seems the first would the option we should take. This is also what hhvm does, according to https://3v4l.org/IdSBu.