Can someone explain how PHP implements associative arrays? What underlying data structure does PHP use? Does PHP hash the key and store it in some kind of hash map? I am curious because I was wondering what the performance of associative arrays where when inserting and searching for keys.
asked Oct 29 '08 at 16:26
Well, for what it is worth, all PHP arrays are Associative arrays.
answered Oct 29 '08 at 16:28
I'll leave this link for someone else to grind through, but you can view the actual C source for PHP at
answered Oct 29 '08 at 16:48
It's all hash tables, according to sources in various web forums: http://www.usenet-forums.com/php-language/15348-zend-engine-array-implementation.html
If you want to be sure, read the source, then compile it, but make sure you can trust your compiler (Warning: PDF, and unrelated, but very cool).
answered Oct 29 '08 at 16:52
@EBGreen is correct.
Which gives you some interesting performance problems, especially when treating an array as a list and using the  (array add) operator. PHP doesn't seem to cache the largest numeric key and add one to it, instead it seems to traverse all of the keys to find what the next numeric key should be. I've rewritten scripts in python because of PHP's dismal array-as-a-list performance.
Associative arrays have the standard dict/hash performance overhead.
answered Oct 29 '08 at 17:14
Are you sure about this? I've just ran benchmarks on a test array of 1000 entries (copying to a new array, one by one), and if you don't specify the key for the new array, it's consistently 7% faster (on PHP 5.2.6)
Oct 29 '08 at 21:08
It's a hash table. The type declaration and hashing function are here: http://cvs.php.net/viewvc.cgi/ZendEngine2/zend_hash.h?view=markup
There is a light weight array and a linked list within the spl (standard php lib)
answered Apr 22 '10 at 18:54
（実装に使われてるのは）ハッシュ表だよ。型の宣言、ハッシュ関数は http://cvs.php.net/viewvc.cgi/ZendEngine2/zend_hash.h?view=markup にあるよ。