Array benchmark:

< ?php
$max = 100000;
$base_memory = memory_get_usage();
$data = array();
for($i = 1; $i <= $max; $i++) {
	$data[] = $i;
	if($i % 100 == 0)	// Having this code in the loop appears to have no significant effect on memory usage
		echo $i."\t".(memory_get_usage() - $base_memory)."\n";
}

Linked list benchmark:

< ?php
class Item {
	public $data;
	public $prev;
	public function __construct($data, $prev) {
		$this->data = $data;
		$this->prev = $prev;
	}
}

$max = 100000;
$base_memory = memory_get_usage();
$head = new Item(1, null);
$tail = $head;
for($i = 2; $i <= $max; $i++) {
	$tail = new Item($i, $tail);
	if($i % 100 == 0)
		echo $i."\t".(memory_get_usage() - $base_memory)."\n";
}

On the other hand, your linked list is contaminated with hashtables as well – every single one of your items is a hashtable. Intuitively, that should lose the efficiency race – so why did it win?

It took me until this point to see it, but you’ve actually filled your array with hashtable nodes, thus burdening your array with all of the inefficiencies of your linked list implementation on top of its own implementation. Not doing arrays justice indeed!

I thought that smooth line was suspect, so I ran my own benchmarks:
http://www.freeimagehosting.net/7kvmr

The line for linked lists cuts off about midway because… my Apache Server crashed at that point. At that point, the linked list is showing memory usage that’s worse by a factor of three.

A true linked list (with no hashtable contamination) would be able to outperform PHP’s array, but a true array would still beat it by a factor of about two. Linked lists really come into their own when you want to insert or remove elements in the middle of the sequence – for arrays, or just about any other data type, this is a linear-time operation, but linked lists can do it in constant time. PHP arrays, being a hybridized linked list, should theoretically be able to pull that off, but as far as I know there is no PHP function that exposes that functionality. A similar situation arises when you want to insert or remove elements at the beginning of the sequence – while there are several data types that can do that in constant time (for example, deques), PHP arrays are not one of them, at least when used conventionally; things may be a bit different when the array is in “associative mode”.