While preparing my "PHP Under The Hood" talk for the Dutch PHP Conference there was a question on IRC about extension_loaded() being faster than function_exists(), which might be strange as both of them are simple hash lookups and a hash lookup is said to be O(1). I started to write some slides for it but figured out that I won't have the time to go through it during that presentation, so I'm doing this now.

He talks about array storage (a "real" array), numeric and string-based keys, the internals of how each is stored and how the differences make the one function faster than the other (hint: it's all about collisions).