The internal representation of a PHP variable container (called zval), contains the type and value of a variable, but also whether it is a reference and what its refcount is. Due to PHP's copy-on-write policy, one specific zval container can be used by multiple variables at the same time as we will see in a bit.

He talks about what the "refcount" field of a variable means and some simple examples showing a single reference, more than one symbols and how PHP handles a "split upon assignment". He also mentions Xdebug's method xdebug_debug_zval that takes in a variable name rather than the variable itself.