PHP: Measure size in kilobytes of a object/array?

• What's an appropriate way of measure a PHP objects actual size in bytes/kilobytes?

Reason for asking:
I am utilizing memcached for cache storage in my web application that will be used by non-technical customers. However, since memcached has a maximum size of 1mb , it would be great to have a function set up from the beginning that I can be used to measure size of selected objects/arrays/datasets, to prevent them from growing to big.

Note that I am only planning on using this as a alert/diagnostic tool to keep track of the cache performance and storage possibilities over time. I assume that calculating speed on each memcached set/add call would slow down things a bit.

I am also aware of storing big datasets in memcached takes away the whole idea of storing things in the RAM, and that is exactly why I need to know in beforehand to prevent customers building up to big datasets.

Thanks a lot