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function get_blog_creator_id($blog_id) {
    global $wpdb;
​
    $blog_id = (int) $blog_id;
    
    if($blog_id !== 0) {
        $email = $wpdb->query("SELECT email FROM {$wpdb->prefix}registration_log WHERE blog_id = {$blog_id}");
        $user = get_user_by( 'email', $email );
        
        return $user->ID;
    }
}