In his latest post Nikita Popov gives a detailed look at PHP's new hashtable implementation and what kinds of improvements it offers over the previous methods. The "hashtable" handling is how the language references array values created during the execution of a script.
About three years ago I wrote an article analyzing the memory usage of arrays in PHP 5. As part of the work on the upcoming PHP 7, large parts of the Zend Engine have been rewritten with a focus on smaller data structures requiring fewer allocations. In this article I will provide an overview of the new hashtable implementation and show why it is more efficient than the previous implementation.
He starts with an introduction to the concept of hashtables, describing them as "ordered dictionaries" of key/value pairs that (internally) reference values in an array. He looks at the old method PHP used to make these links and how the new version, with the help of zval handling, is different. He talks about how it handles the order of elements, does lookups and the introduction of "packed" and "empty" hashtables. He ends the post with a look at this new implementation's memory utilization and what kind of performance gains we can expect with its introduction in PHP7.