PHP variable / function / class names using special characters

I understand that the underscore _ is an acceptable character for naming variables / functions / classes etc. However I was wondering if there are any other special characters which can be used. I tested out a few with no luck and have assumed for a long time that there are not, however I figured I would see if anyone else knows for certain. This would be mostly for aesthetic purposes, however I imagine a special character naming convention would be useful when working with other developers to define value types etc.