Uncaught TypeError: unserialize(): Argument #1 ($data) must be of type string

I discovered this error too and found it was related to my OPcache plugin Docket Cache. If I disable OPcache then Nested Pages starts working again. I didn’t have this problem in PHP 7.4 but it appeared after updating to 8.1. It’s unfortunate because I want to use OPcache for the overall performance of my site, but Nested Pages makes managing the site so much easier.

WordPress version 6.2
Active theme: Theme Name (version 17.0)
Current plugin: Nested Pages (version 3.2.3)
PHP version 8.1.17



Error Details
=============
An error of type E_ERROR was caused in line 39 of the file /path/to/site/wp-content/plugins/wp-nested-pages/app/Entities/Listing/ListingRepository.php. Error message: Uncaught TypeError: unserialize(): Argument #1 ($data) must be of type string, array given in /path/to/site/wp-content/plugins/wp-nested-pages/app/Entities/Listing/ListingRepository.php:39
Stack trace:
#0 /path/to/site/wp-content/plugins/wp-nested-pages/app/Entities/Listing/ListingRepository.php(39): unserialize(Array)
#1 /path/to/site/wp-content/plugins/wp-nested-pages/app/Entities/Listing/Listing.php(317): NestedPages\Entities\Listing\ListingRepository->visiblePages('page')
#2 /path/to/site/wp-content/plugins/wp-nested-pages/app/Entities/Listing/Listing.php(392): NestedPages\Entities\Listing\Listing->listOpening(Array, 0)
#3 /path/to/site/wp-content/plugins/wp-nested-pages/app/Entities/Listing/Listing.php(369): NestedPages\Entities\Listing\Listing->listPostLevel()
#4 /path/to/site/wp-content/plugins/wp-nested-pages/app/Views/listing.php(98): NestedPages\Entities\Listing\Listing->getPosts()
#5 /path/to/site/wp-content/plugins/wp-nested-pages/app/Entities/Listing/Listing.php(280): include('/home/soulartst...')
#6 /path/to/site/wp-includes/class-wp-hook.php(308): NestedPages\Entities\Listing\Listing->listPosts('')
#7 /path/to/site/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters('', Array)
#8 /path/to/site/wp-includes/plugin.php(517): WP_Hook->do_action(Array)
#9 /path/to/site/wp-admin/admin.php(259): do_action('toplevel_page_n...')
#10 {main}
  thrown

@plasmax I have a potential fix here… I fed the error message and the file /app/Entities/Listing/ListingRepository.php into ChatGPT about it and asked it to resolve the incompatibility with PHP8. In approximately 5 seconds it wrote back:

The problem is happening in the visiblePages($post_type) function when trying to unserialize() an array. If the user meta ‘np_visible_posts’ is not ‘1’, the code attempts to unserialize the value without confirming it’s a string.

We can resolve this issue by first checking if the data returned from get_user_meta() is a serialized string or an array before attempting to unserialize it. Here’s how you can modify the visiblePages($post_type) method:

/**
* User's Toggled Pages
*/
public function visiblePages($post_type)
{
    $meta = get_user_meta(get_current_user_id(), 'np_visible_posts', true);
    if ( $meta == '1' ) return [];
    if ( is_serialized($meta) ) {
        $visible = unserialize($meta);
    } elseif ( is_array($meta) ) {
        $visible = $meta;
    } else {
        $visible = [];
    }
    if ( !isset($visible[$post_type]) ) $visible[$post_type] = [];
    return $visible[$post_type];
}

This code first checks if the get_user_meta() result $meta is ‘1’. If so, it returns an empty array. If not, it then checks if $meta is a serialized string with the is_serialized() function. If it is, it unserializes the string into the $visible variable. If $meta is already an array, it assigns it to the $visible variable. If $meta is neither a serialized string nor an array, it assigns an empty array to $visible. It then checks if $visible[$post_type] is set, if not it assigns an empty array to it, and finally returns $visible[$post_type].

This change should fix the TypeError that’s been raised in your application.

All I had to do after that was to disable Nested Pages, comment out the original visiblePages($post_type) function in /app/Entities/Listing/ListingRepository.php and paste in the replacement, and then re-enable Nested Pages. It seems to be working again. I haven’t experienced the error again.

It’s not perfect because it doesn’t seem to remember the toggle state of my nested pages list anymore when I leave the All Pages list and come back to it, but at least it doesn’t crash.