[PHP/WordPress] Why does this code work?

Fenster

For my current client I need to get the ID of the ultimate parent of a category. The Bestest Way to do this is to iteratively call get_category() on successive parents until you hit the top category, but being bored and having a pint in me I decided to use a recursive function.

This function works-it correctly returns the top category ID-but logic tells me that the function should be returning an error, null or 0. This is a quirk of either WordPress or PHP, and I would like to better understand why this function works, because I feel this will teach me a fundamental lesson about PHP:

https://gist.github.com/bhalash/c6c8e9eed7bdc4c87cb8

function get_category_parent_id($cat_id) {
    $category = get_category($cat_id);
    $parent = $category->category_parent;

    if ($parent) {
        get_category_parent_id($parent);
    } 

    return $parent;
}

What I expect to happen is that I will eventually pass null as a parameter to get_category, which will return an error, so when I look at the output of this function I expect to see either null or an error object.

I say this because I assign the value to the parent before I use it, so I eventually assign an error object or null to the value and then return it.

bpmurray

Fenster wrote: »

What I expect to happen is that I will eventually pass null as a parameter to get_category

Why would you expect that? It explicitly checks that the $parent is not null, so it never passes that to the function.

FobleAsNuck

if $parent==null then this block will not run

Fenster

1. I fetch a category based on $cat_id.
2. I look at the object, get category_parent and assign it to $parent.

When a category has no parent, category_parent returns false.
I have now assigned a value of false to $parent, skip the loop and return false.

Either way, I expect a return value from the overall function of null, false or error.

Better explanation:

Grandchild ID: 192
Child ID: 137
Parent ID: 101

Level #0:
$cat_id = 192
$parent = 137

$parent is true, so go to level #1.

Level #1:
$cat_id = 137
$parent = 101

$parent is true, so go to level #2.

Level #2:
$cat_id = 101
$parent = 0/false

$parent is false, but has a value of false, so return $parent, with a value of false.

Again, I coded this pretty ****tily and I don't understand why it returns, say, the parent ID of 101 instead of false/null/error.

Talisman

Did you look at the get_category_parents function?

Fenster

Yeah. I'm still not sure why the original worked, but here's the non-drunk version of the function:

function find_category_parent_id($cat_id = null) {
    $category = get_category($cat_id); 

    if ($category->category_parent) {
        $cat_id = find_category_parent_id($category->category_parent);
    }

    return $cat_id;
}

Thank you all for your answers!