PHP deprecation warning in /wp-includes/general-template.php

[nexbridge]

I'm getting the following warning on what is currently the latest version of WordPress:

PHP Deprecated:  explode(): Passing null to parameter #2 ($string) of type string is deprecated in /wp-includes/general-template.php on line 1425

This is the line in question:

$title_array = apply_filters( 'wp_title_parts', explode( $t_sep, $title ) );

The PHP version is 8.2.

[Tussendoor]

We are experiencing the same problem. It occurs on a dynamic post which we are creating on runtime.

Ways to reproduce:

1. Add a rewrite tag to be able to catch a custom query

   <?php
   add_rewrite_tag('%rewrite_tag_example%', '([^&]+)');
   

2. Add a rewrite rule for the rewrite tag

   <?php
   add_rewrite_rule('basepage/([^/]*)/?', 'index.php?rewrite_tag_example=$matches[1]', 'top');
   

3. Register a template with template_include

   <?php
   add_filter('template_include', 'registerFrontendTemplate', 9999);
   

4. Add the registerFrontendTemplate function with a custom query to dynamically populate a template

   <?php
   /**
    * Register the template file
    *
    * @param string $template
    * @return string
    */
   function registerFrontendTemplate(string $template)
   {
       if (empty(get_query_var('rewrite_tag_example'))) {
           return $template;
       }
   
       // Add custom query here. To get data from a custom database table for 
       // example. Use the custom query variable to get postdata dynamically 
       // based on the request
   
       return locate_template('template/single.php');
   }
   

5. Refresh the permalinks by navigating to the permalinks settings in the WordPress admin.

6. Go to the generated frontend page: https:/example.com/basepage/test. With that URL the "rewrite_tag_example" query variable is filled with "test".

7. On this page the <title> HTML-element will show the deprecation warning. This is because in wp-includes/general-template.php the wp_title function call the single_post_title function via:

   <?php
   // If there is a post.
   if ( is_single() || ( is_home() && ! is_front_page() ) || ( is_page() && ! is_front_page() ) ) {
       $title = single_post_title( '', false );
   }
   

And the single_post_title function returns null because of:

<?php
if ( ! isset( $_post->post_title ) ) {
   return;
}

Possible solution
We could make the single_post_title function return a string value. But this does not solve the problem for other possible occurances of this error that are possibly triggered by the wp_title function. So that requires changing the functions below to always return a string instead of string|void:

• single_post_title
• post_type_archive_title
• single_term_title
• single_term_title
• post_type_archive_title

As the default value of $title is already an empty string (and not null) in the wp_title function we could try this for each occurance of those functions in wp_title:

<?php
// If there is a post.
if ( is_single() || ( is_home() && ! is_front_page() ) || ( is_page() && ! is_front_page() ) ) {
    $title ??= single_post_title( '', false );
}

The Null Coalescing Assignment operator (??=) does require PHP 7.4 tho.

Hope this helps! :)

[michaelreetz]

Perhaps it would be better to just do the Null coalescing operator inside the explode.

<?php
/**
 * Filters the parts of the page title.
 *
 * @since 4.0.0
 *
 * @param string[] $title_array Array of parts of the page title.
 */
$title_array = apply_filters( 'wp_title_parts', explode( $t_sep, $title ?? '') ); 

This feels to me like what the code would have done in previous versions. And it would be compatible to php7.0.

[Tussendoor]

Replying to michaelreetz:

Yes that seems like a good approach!

[nexbridge]

Just to say that this is still happening in the latest version (6.5.5). I notice that the ticket status is currently "awaiting review" - how long does it normally take for a ticket to be reviewed?

[nexbridge]

Just to say that this is still happening in 6.6.1, however the line number has now changed to 1440.

[nexbridge]

Patch

[nexbridge]

I've now added a patch as per the suggestion by @michaelreetz.

[o----o]

Same here.
The problem occurs at least in my case when on custom route where there is no "title" parameter as it is on default WP views.

WP Version 6.6.2

[Beee]

Still happening in 6.7.

When will this fix be applied ? This error is kind of annoying.

[nexbridge]

Confirmed that this is still happening in 6.7.1, and the line number is now 1439.