How can one ensure that session IDs are not started when a search engine visits a PHP forum?

To ensure that session IDs are not started when a search engine visits a PHP forum, you can check the user agent of the incoming request and only start the session if the user agent does not belong to a search engine bot. This can help prevent unnecessary session creation for search engine crawlers.

if(!preg_match('/bot|crawl|slurp|spider/i', $_SERVER['HTTP_USER_AGENT'])) {
    session_start();
}