如何确定上下级别之间的级别数？

I have a function checkLevel($upper,$lower). Suppose I pass checkLevel(2,4). Now 2 is the parent of 3 and 3 is the parent of 4. The gap between them is of 2. Similarly, if I enter checkLevel(1,4), the result should be 3 because 1 is the parent of 2, 2 is the parent of 3 and 3 is the parent of 4. The number of levels are 3. Please see the image below. Can any one help me out?

It should work.

$level = 0;
function checkLevel($start,$end)
{
    global $level;
    $level++;
    $array = array(0=>array('id'=>1,'parent_id'=>'0','test'=>'a'),1=>array('id'=>2,'parent_id'=>'1','test'=>'b'),2=>array('id'=>3,'parent_id'=>'2','test'=>'c'),3=>array('id'=>4,'parent_id'=>'3','test'=>'d'));
    $key = array_search($end,array_column($array, 'id'));

    if($key == $start)
        return;
    else
        checkLevel($start,$key);
    //echo $key;exit;

}

checkLevel(2,4);
echo $level;