crypting

Crypt private messages - wich can't be read by server owner / developer

Iam not sure if this is the right place but lets see. Iam developing an system to send private messages from one user to an other use. Under the hood runs PHP 5.2.x . Now iam looking for any way to crypt private messages in a way that i cant access them. In the moment i have no idea how to realize such a system, where iam not knowing ...