Consider isDescendantOf($x) or hasDescendant($x) or isAncestorOf($x) or hasAncestor($x) or similar to make intent more clear -- I think this $x->isDescendant($y) can be reasonably be read as either "$x is a descendant of $y" or "$y is a descendant of $x".

Specifically, I would expect this to mean "$x is a descendant of $y", because most $x->is...() methods return information about $x, but it actually means "$y is a descendant of $x". I would perhaps call this method isAncestorOf($y). This makes the $x/$y relationship more clear, although I don't think X being an ancestor of itself is always intuitive (but I don't have a better term for it offhand).