Must Use Plugin Causing Query Error

Question

I created an MU Plugin which will only display the posts from certain tags in the loop like this:

function custom_tags( $query ) {
    $query->set( 'tag', array( 'custom', 'general' ) );
}
add_action( 'pre_get_posts', 'custom_tags' );

It works fine when I remove the array and only check for 1 tag, but how do I get it to work with more than 1 tag like I'm trying to above?

The Error I'm getting is:

Warning: strpos() expects parameter 1 to be string, array given in /srv/users/s/wp-includes/query.php on line 1966

Warning: preg_split() expects parameter 2 to be string, array given in /srv/users/s/wp-includes/query.php on line 1967

Updated Code:

$current = substr($_SERVER[HTTP_HOST], 0, -4);

function custom_tags( $query ) {
        $query->set( 'tag', 'general,{$current}' );
}
add_action( 'pre_get_posts', 'custom_tags' );

Answer 1

As Milo ( and your errors ) point out: you're passing an array where a string is expected. According to WP_Query tag parameter

Show posts associated with certain tags.

• tag (string) - use tag slug.

To get around this you just need to pass a comma separated string:

function custom_tags( $query ) {
    $query->set( 'tag', 'custom,general' );
}
add_action( 'pre_get_posts', 'custom_tags' );

Answer 2

Create a proper tax_query, for example:

'tax_query' => array(
    array(
        'taxonomy' => 'people',
        'field'    => 'slug',
        'terms'    => 'bob',
    ),
),

But your code "update code" is going to fail for other reasons too.

1. $current is out of scope.
2. And your variable won't expand inside single quotes

The bare bones changes you need are:

function custom_tags( $query ) {
  $current = substr($_SERVER[HTTP_HOST], 0, -4);
  $query->set( 'tag', "general,{$current}" );
}
add_action( 'pre_get_posts', 'custom_tags' );

But as mentioned, I'd create a proper tax_query

function custom_tags( $query ) {
  $current = substr($_SERVER[HTTP_HOST], 0, -4);
  $tax = array(
    array(
        'taxonomy' => 'tag',
        'field'    => 'slug',
        'terms'    => $current,
        'operator' => 'IN' // This is default
      ),
    );
  $query->set( 'tag', "general,{$current}" );
}
add_action( 'pre_get_posts', 'custom_tags' );

The operator can be changed to get different behavior:

operator (string) - Operator to test. Possible values are 'IN', 'NOT IN', 'AND', 'EXISTS' and 'NOT EXISTS'. Default value is 'IN'.

And your code is going to run on every query on the site, just about. That is going to alter a lot of things, and will certainly break things. You need to restrict it to only those locations you need it. I don't know exactly where or when this is supposed to run but this should be a start:

function custom_tags( $query ) {
  if (is_admin() 
    || $query->is_main_query()
  ) {
    return;
  }
  $current = substr($_SERVER[HTTP_HOST], 0, -4);
  $tax = array(
    array(
        'taxonomy' => 'tag',
        'field'    => 'slug',
        'terms'    => $current,
        'operator' => 'IN' // This is default
      ),
    );
  $query->set( 'tag', "general,{$current}" );
}
add_action( 'pre_get_posts', 'custom_tags' );