The Standard PHP Library offers PHP developers some excellent tools that, in some cases, can make quite a bit of performance difference than some of their normal counterparts in the rest of the language. In this post from Matt Butcher, two features are compared - normal arrays and the SplObjectStorage feature of the SPL.
One of my projects, QueryPath, performs many tasks that require maintaining a set of unique objects. In my quest to optimize QueryPath, I have been looking into various ways of efficiently storing sets of objects in a way that provides expedient containment checks. [...] Recently I narrowed the list of candidates down to two methods: Use good old fashioned arrays to emulate a hash set or use the SPLObjectStorage system present in PHP 5.2 and up.
He works through the comparison, showing how each of them can emulate the hashed set with the goal of being able to easily iterate and simple to search. The result is that PHP's normal arrays best the SplObjectStorage object in his benchmarking (code included).