Assigning alternate single-{cpt} template based on blog_id in multisite

Question

I have a multisite network with some shared custom post types, and I'm trying to create a function that loads a custom single-{cpt} template for a given post type by $blog_id when defined, and otherwise fall-back to the default single- template. In this case, the custom post type is resource, which would default to single-resource.php and I'm setting a variable $custom_resource_single to the applicable template file name as follows:

if ($blog_id == 5) {
   $custom_resource_single = 'single-resource-alt.php';
}

I'm then trying to pass this variable to a function used as a callback in the template_include filter hook as follows (adapting somewhat from this thread), but it's not working:

function custom_resource_single($custom_resource_single, $template) {
    if ( is_singular('resource') && !empty($custom_resource_single) ) {
        $custom = locate_template($custom_resource_single);
        return $custom;
    }
    return $template;
}
add_filter( 'template_include', function() use ($custom_resource_single, $template) { custom_resource_single($custom_resource_single, $template); });

I attempting to use a closure based on some other threads I've found here, but I'm clearly not using it properly. I can successfully achieve the result by passing an anonymous function with the $blog_id conditional directly to template_include hook, but I'd really like to be able to pass a variable ($custom_resource_single) to the function that is defined elsewhere.

Please let me know if my aim as described above is unclear, and thanks in advance for any assistance.

Answer 1

Actually, I noticed that your closure is not capturing/using the proper $template variable which is passed from the template_include filter hook:

// This hook is defined in wp-includes/template-loader.php
$template = apply_filters( 'template_include', $template )

The closure is also not returning the template..

So your closure should look like:

add_filter( 'template_include', function( $template ) use ( $custom_resource_single ) {
    return custom_resource_single( $custom_resource_single, $template );
} );

But you may want to pass the $custom_resource_single by reference (use ( &$custom_resource_single )) in case it gets changed after the above closure:

if ( $blog_id == 5 ) {
    $custom_resource_single = 'single-resource-alt.php';
}

add_filter( 'template_include', function( $template ) use ( &$custom_resource_single ) {
    return custom_resource_single( $custom_resource_single, $template );
} );

// $custom_resource_single is changed here.
if ( /* condition */ ) {
    $custom_resource_single = 'file.php';
}