How to change the title on a WordPress page?

Question

Context: WordPress 5.4.5, Yoast 3.7.1

I'm a plugin developer who has access to the client's site. The site has Yoast 3.7.1 installed and I'm wondering if that is significant because no matter what I do I can't change the 404 page's title.

Now on other pages on StackOverflow where similar questions have been posed (here, here and here for example), those answering have asked if the header.php is correctly embedding a call to wp_title(). Here's what's in the current theme's header.php at that point:

    <title><?php wp_title( '|', true, 'right' ); ?></title> 

Interestingly, in my 404.php page, wp_get_document_title() tells me that the document title is Page not found - XXXX even though the wp_title call above specifies the separator as |. Yoast's rewriting of titles has been disabled so I'm not at all sure where that dash is coming from.

My plugin does a REST call and pulls in content from off-site for inclusion in the page. Part of that content is the text to be used in the title.

On previous client sites, I've been able to do the following:

add_filter('wp_title', 'change_404_title');
function change_404_title($title) {
    if (is_404()) 
    {
        global $plugin_title;
        if (!empty($plugin_title)) 
        {
             $title = $plugin_title;
        }
    }
    return $title;
}

However, on this site, that's not working.

I have tried, based on the version of WordPress being used, hooking the pre_get_document_title filter, viz

add_filter('pre_get_document_title', 'change_404_title');

but again to no avail. I am currently reading up on Yoast ...

Answer 1

wp_title deprecated since version 4.4. So we should use the new filter pre_get_document_title. Your code looks fine but I am confused about global $plugin_title. I would rather ask you to Try this first

add_filter('pre_get_document_title', 'change_404_title');
function change_404_title($title) {
    if (is_404()) {
        return 'My Custom Title';
    }
    return $title;
}

If it doesn't work then try changing the priority to execute your function lately.

add_filter('pre_get_document_title', 'change_404_title', 50);