It's pretty close. It does get around the 0 (match at first char)/false (no match) confusion that sometimes causes bugs with strpos, though. You could state this more succinctly as:
function inStr($needle, $haystack)
return strpos($haystack, $needle) !== false;
PS: More comments are not always better comments. If anyone needs guidance as to what 'return true' does, noting that it returns true isn't going to clarify it any.