Checking whether post_parent = 0 in WordPress - php

Question

I am creating a site using WordPress using a theme which has this PHP at the top of each page and I am confused as to what it does.
<?php
global $post;
global $wp_query;
if ( $post->post_parent != 0 ) {
$thePostID = $post->post_parent;
} else {
$thePostID = $wp_query->post->ID;
};
?>
I was just wondering if anyone could explain exactly what this does? I think it checks to see if the post_parent id is 0 which isn’t allowed in WordPress and sets the post id to the post_parent but I’m not 100% sure.

Answer 1

I think it checks to see if the post_parent id is 0 which isn’t allowed in WordPress
The $post->post_parent is allowed to be 0. If the value is 0, it simply means that the page is a top level page.
A page that has a $post->post_parent other than 0, is a child of another page.
For example, take this page structure as an example:
id page_title post_parent
1 Home 0
2 About 0
3 Staff 2
4 History 2
5 Contact 0
The resulting page/menu structure would be:
Home
About
Staff
History
Contact
The code in question:
if ($post->post_parent != 0) {
$thePostID = $post->post_parent;
} else {
$thePostID = $wp_query->post->ID;
}
I'm not sure why your theme might have the code, but a possible reason might be to get a menu related to the current page. If you're viewing the top level page (i.e. $post->post_parent == 0), then it would show all child pages, or if you're viewing a sub page, the menu might show all sibling pages.
A sample menu generated using this method
Add this to your functions.php file so it's accessible throughout the theme.
/**
* Get top parent for the current page
*
* If the page is the highest level page, it will return its own ID, or
* if the page has parent(s) it will get the highest level page ID.
*
* #return integer
*/
function get_top_parent_page_id() {
global $post;
$ancestors = $post->ancestors;
// Check if the page is a child page (any level)
if ($ancestors) {
// Get the ID of top-level page from the tree
return end($ancestors);
} else {
// The page is the top level, so use its own ID
return $post->ID;
}
}
Add this code to your theme where you want to display a menu. You will need to customise it to suit your particular needs, but it gives you an example of why someone might use the code you asked about.
// Get the highest level page ID
$top_page_id = get_top_parent_page_id();
// Display basic menu for child or sibling pages
$args = array(
'depth' => 1,
'title_li' => FALSE,
'sort_column' => 'menu_order, post_title',
'child_of' => $top_page_id
);
echo wp_list_pages($args);

Answer 2

So as far as i see, this snippet does the following:
Loads the global post object.
Loads the global query object.
Checks (or tries to check) if the current post has a parent filter (?).
If it has, it sets the variable $thePostID to the id of the post_parent parameter of the current post object.
If it hasnt, it sets the post id in the wp_query to the id of the current post.
All together, this seems very unnecessary, bad performing and strange Oo.
Does ist work actually? ^^
I would advise you on using another theme if possible ;-)