Detailed explanation of return value of infinite classification recursive function in ecshop

When doing product classification in the secondary development of ecshopindex, the top-level category id must be obtained based on the category id. The first reaction was to use recursion to pass it out, so I wrote the recursive function as follows:

function getCatTopId($cat_id)
{
    if ($cat_id)
    {
      $res = Array();
        $sql = 'SELECT cat_id, parent_id'
             . ' FROM ' . $GLOBALS['ecs']->table('category') 
             . ' WHERE cat_id = ' . $cat_id . ' AND is_show = 1';
        $res = $GLOBALS['db']->getAll($sql);
        if ($res[0]['parent_id'] > 0)
        {
            getCatTopId($res[0]['parent_id']);
        }
        else
        {
            return $res[0]['cat_id'];
        }
    }
    else
    {
        return 1;
    }
}

A test program, no return value? After checking for a long time, no error was found. It seemed that the circuit in the brain was broken. When I asked Shui Shen (a kind-hearted netizen) today, he helped me answer it, and the modification is as follows:

function getCatTopId($cat_id)
{
    if ($cat_id)
    {
     $res = Array();
        $sql = 'SELECT cat_id, parent_id'
             . ' FROM ' . $GLOBALS['ecs']->table('category') 
             . ' WHERE cat_id = ' . $cat_id . ' AND is_show = 1';
        $res = $GLOBALS['db']->getAll($sql);
        if ($res[0]['parent_id'] > 0)
        {
            return getCatTopId($res[0]['parent_id']); // 修改处，多写个 return ，让函数返回值
        }
        else
        {
            return $res[0]['cat_id'];
        }    }
    else
    {
        return 1;
    }
}

The function is written internally. Even if it returns, it only returns to the position of the internal function, so there is still There is a main function, which must be returned again

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