Only variables should be passed by reference

Hi @babelhalsupport, thanks for providing the site diagnostic and necessary details.

Similar to the topic you posted related to unlink(), the PHP notice is designed to be ignored. For the sake of anybody searching out this topic at a later date, I’ll quote what I said there:

Errors such as this are often picked up in Query Monitor (or in this case your custom mu-plugin) for our intended silent failures. We sometimes utilize WordPress’ $wpdb->suppress_errors(true); function or PHP’s @ in situations we know an error may occur. These tools tend to ignore those whereas they’d normally not show to any customers or their site’s visitors.

There’s no fixed amount of resources we know Wordfence will use on each customer’s server. Therefore, some non-critical situations where it’s more expensive on resources to retry or check for the presence of a file/record first, we simply choose to fail silently and continue.

Many thanks again,
Peter.