Error: array_map(): Argument #2

Question

Once I've updated WordPress to version 5.3 with this error message:

[17-Nov-2019 01:15:14 UTC] PHP Warning:  array_map(): Argument #2 should be an array in /home/name/public_html/web/wp-includes/class-wp-query.php on line 2069
[17-Nov-2019 01:15:14 UTC] PHP Warning:  implode(): Invalid arguments passed in /home/name/public_html/web/wp-includes/class-wp-query.php on line 2069
[17-Nov-2019 01:15:14 UTC] WordPress database error You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ') AND ( 
  wpuw_term_relationships.term_taxonomy_id IN (9)
) AND ( 
  wpuw_postm' at line 1 for query SELECT SQL_CALC_FOUND_ROWS  wpuw_posts.ID FROM wpuw_posts  LEFT JOIN wpuw_term_relationships ON (wpuw_posts.ID = wpuw_term_relationships.object_id) INNER JOIN wpuw_postmeta ON ( wpuw_posts.ID = wpuw_postmeta.post_id ) WHERE 1=1  AND wpuw_posts.ID NOT IN () AND ( 
  wpuw_term_relationships.term_taxonomy_id IN (9)
) AND ( 
  wpuw_postmeta.meta_key = 'ecpt_toparticle'
) AND wpuw_posts.post_type = 'post' AND (wpuw_posts.post_status = 'publish' OR wpuw_posts.post_status = 'private') GROUP BY wpuw_posts.ID ORDER BY wpuw_posts.post_date DESC LIMIT 0, 5 made by require('wp-blog-header.php'), require_once('wp-includes/template-loader.php'), include('/themes/mydesign/category.php'), WP_Query->__construct, WP_Query->query, WP_Query->get_posts

In category.php code:

<?php
$args = array(
    'meta_key'=> 'ecpt_toparticle',
    'showposts' => 5,
    'category__in' => $cat,
    'post__not_in' => $tag
);
$sticky_query = new WP_Query( $args );
while ( $sticky_query->have_posts() ) : $sticky_query->the_post();
?>

It also doesn't work with the Select tag:

<option value="?tag=usa" <?php if ($tag == usa) echo 'selected="selected" ';?>>Usa</option>

The only solution I made so far was to revert back to WordPress version 5.2.4.

Answer 1

(Revised answer, based on the code in question and this)

After the discussion, I realized that:

1. You should replace the post__not_in with tag; i.e. use 'tag' => $tag.

   You should also understand that post__not_in should be an array of post IDs and not tag/category slugs, names, etc. And if you don't pass an array, then you'd get the error in question, even in WordPress version 5.2.4. :)

2. You should use get_query_var( 'tag' ) to get the selected tag (slug).

3. And remove the category__in.

So your WP_Query code would look like so:

$args = array(                 // then the query args
    'posts_per_page' => 5, // you should use posts_per_page and not showposts
    'post_type'      => 'post',
    'tag'            => get_query_var( 'tag' )
);

$sticky_query = new WP_Query( $args );

Additionally:

1. The select code (for letting users pick a tag):

   <?php $tag = get_query_var( 'tag' ); ?>
   <select name="formal" class="city-choice" onchange="handleSelect(this)"
       data-action="<?php echo esc_url( add_query_arg( 'tag', '%tag%' ) ); ?>">
       <option value="">Select a tag</option>
       <option value="usa" <?php selected( $tag, 'usa' ); ?>>Usa</option>
       <option value="italy" <?php selected( $tag, 'italy' ); ?>>Italy</option>
   </select>
   

2. The JavaScript code (for submitting the tag selected via the above select):

   <script type="text/javascript">
   function handleSelect(elm) {
       window.location = elm.dataset.action.replace( '%tag%', elm.value );
   }
   </script>