I'm using PDO for database handling and I recently started using persistent database connections to (according to the php.net documentation) save resources. I'm not, however, 100% sure when it is bad to be using persistent database connections. Does anyone know the performance and security implications of using a persistent connection or just a situation where it is bad to have a persistent connection?

I remember reading something about them keeping the connection open, so on a high-usage site, you could run out of available connections.