Mixed results with is_page() WordPress function, when using $this, or self

Question

I'm converting an old plugin from PHP5 -> PHP8 and I came across this check, which caused an error when executing:

Notice: Accessing static property PropertySearch::$listing_details_template_name as non static

I found online that this can be fixed, by using self instead of $this to access a static member. However, when I pass self to the is_page Wordpress function, it returns false, where-as when I pass $this, it returns true.

From what I can tell, both of these contain the exact same string value.

• self::$listing_details_template_name
• $this->listing_details_template_name

One thing I noticed while debugging, is that upon entering the is_page function, the $page variable is NULL when using $this->listing_details_template_name.

When entering the is_page function with self::$listing_details_template_name, the $page variable contains the name of the template.

Here's a quick demo of what I'm talking about.

Ultimately, I'm trying to find out why this is happening, and how I can pass the static member to is_page to get the expected result of true.

global $wp_query;

    if (is_page(self::$listing_details_template_name)) {
        echo nl2br ("[DEBUG] - self2 = " . self::$listing_details_template_name . " \r\n");
    }

    if (is_page($this->listing_details_template_name)) {
        echo nl2br ("[DEBUG] - this2 = " . self::$listing_details_template_name . " \r\n");
    }

===

For the time being, I've added this to the class-wp-query.php file of WordPress.

    if ($page = 'property-search-single-property-details-template.php') {
        $page = NULL;
    }

However, this is just a bandaid so I can continue debugging. I would love a solution that does NOT involve me modifying the core WordPress files. Or at the very least, some further understanding of why this is happening.

===

    class PropertySearch {

    private static $listing_details_template_name;

    public function __construct() {

        if (self::IsSingleProperty()) {
            self::$listing_details_template_name = 'property-search-single-property-details-template.php';
        } else {
            self::$listing_details_template_name = 'property-search-details-template.php';
        }

    public function PropertySearchListingDetails() {
        global $wp_query;

        if (is_page(self::$listing_details_template_name)) {
            if (!empty($wp_query->query_vars['building'])) {
                return json_decode((string) $this->SyndicationAPI->getDetails($wp_query->query_vars['building']), null, 512, JSON_THROW_ON_ERROR);
            } else {
                return json_decode((string) $this->SyndicationAPI->getDetails(SINGLE_PROPERTY_ID), null, 512, JSON_THROW_ON_ERROR);
                //return 'No listing found';
            }
        } else {
        }
    }

}

Answer 1

You've shared that this is how the member variable is defined:

class PropertySearch {
    private static $listing_details_template_name;

This means it is a private static variable and can be accessed using static accessor such as self::$listing_details_template_name or PropertySearch::$listing_details_template_name.

Additionally, you've declared that it's a private member variable, so it can only be referred to from within the same class via self::.

Why not $this?

$this would be innapropriate because $this refers to a specific object, not a class, and static member variables do not belong to objects, they belong to the definition of a class, specifically PropertySearch.

Remember, PropertySearch is not an object, it's a class, a class is a definition that's used to create instances of objects. I may be human but it would be a mistake to say human::$name = 'Tom'; because that's saying that all humans are named Tom, not just me specifically.

Likewise $this->listing_details_template_name heavily implies that different PropertySearch objects can have different listing detail templates, and that it's a dynamic changing value, not a static value shared by all objects that use that class.

That is why you get this message:

Notice: Accessing static property PropertySearch::$listing_details_template_name as non static

Ironically, if you changed it to be a dynamic member variable it would work with $this and it would no longer be appropriate to use self.

Methods

Instead of directly accessing it why not use a method? Then you can add filtering and other logic.

TLDR

• static member variables are on classes
• dynamic member variables are on objects
• Use $this->variablename or $object->variablename for dynamic member variables
• Use self::$variablename or ClassName::$variablename for static member variables
• Consider using a method as an alternative option