//my id's hierarchy for the example: 0 -> 71 -> 78 -> 80

function parent_id($cat_id) {
	$db = new ps_DB;

	$q = "SELECT * FROM #__{vm}_category_xref ";
	$q .= "WHERE #__{vm}_category_xref.category_child_id='$cat_id' ";
	
	$db->setQuery($q);
	$db->query();
	
	return $db->f("category_parent_id");
}

echo parent(80); //returns 78 -> OK!

function niveau($cat_id,$A){
	echo "$cat_id($A) -> ";
	if(parent_id($cat_id)==0) {
		return $A;
	} else {
		niveau(parent_id($cat_id),$A+1);	
	}
}

echo niveau(80,0); //returns '80(0) -> 78(1) -> 71(2) ->' but i'd like only the '2'