Update: Terry Chay has answered my question about why this is happening. In a nutshell, PHP is using 33,160 opcodes and 33,157 regsters for the verbose code. In comparison, the serialized array only uses 5 opcodes and 2 registers. He used something called VLD, which I had not heard of, to figure all this out.

So, at dealnews, we have a category tree. To make life easy, we dump it to an array in a file that we can include on any page. It has 420 entries. Expanded, one entry may look like:
$CATEGORIES[202]['id'] = "202";

$CATEGORIES[202]['name'] = "clothing & accessories";

$CATEGORIES[202]['parent'] = "0";

$CATEGORIES[202]['standalone'] = "";

$CATEGORIES[202]['description'] = "clothing";

$CATEGORIES[202]['precedence'] = "0";

$CATEGORIES[202]['preferred'] = "0";

$CATEGORIES[202]['searchable'] = "1";

$CATEGORIES[202]['product'] = "1";

$CATEGORIES[202]['aliased_id'] = "0";

$CATEGORIES[202]['path'] = "clothing & accessories";

$CATEGORIES[202]['url_safe_name'] = "clothing-accessories";

$CATEGORIES[202]['child_count'] = "6";

$CATEGORIES[202]['childlist'][0] = 202;

$CATEGORIES[202]['childlist'][1] = 2;

$CATEGORIES[202]['childlist'][2] = 275;

$CATEGORIES[202]['childlist'][3] = 4;

$CATEGORIES[202]['childlist'][4] = 481;

$CATEGORIES[202]['childlist'][5] = 446;

$CATEGORIES[202]['childlist'][6] = 454;

$CATEGORIES[202]['childlist'][7] = 436;

$CATEGORIES[202]['childlist'][8] = 205;

$CATEGORIES[202]['childlist'][9] = 227;

$CATEGORIES[202]['childlist'][10] = 203;

$CATEGORIES[202]['childlist'][11] = 280;

$CATEGORIES[202]['childlist'][12] = 204;

$CATEGORIES[202]['children'][2] = &$CATEGORIES[2];

$CATEGORIES[202]['children'][275] = &$CATEGORIES[275];

$CATEGORIES[202]['children'][4] = &$CATEGORIES[4];

$CATEGORIES[202]['children'][481] = &$CATEGORIES[481];

$CATEGORIES[202]['children'][446] = &$CATEGORIES[446];

$CATEGORIES[202]['children'][454] = &$CATEGORIES[454];

$CATEGORIES[202]['children'][436] = &$CATEGORIES[436];

$CATEGORIES[202]['children'][205] = &$CATEGORIES[205];

$CATEGORIES[202]['children'][227] = &$CATEGORIES[227];

$CATEGORIES[202]['children'][203] = &$CATEGORIES[203];

$CATEGORIES[202]['children'][280] = &$CATEGORIES[280];

$CATEGORIES[202]['children'][204] = &$CATEGORIES[204];

So, I was curious how efficient this was. I noticed that some code that was using this array was jumping in memory usage as soon as I ran the script. So, I devised a little piece of code:
<?phpecho "Memory used: ".number_format(memory_get_usage())." bytesnn";



include_once "./cat_code.php";



echo "type: ".gettype($CATEGORIES)."n";



echo "count: ".count($CATEGORIES)."nn";



echo "Memory used: ".number_format(memory_get_usage())." bytesnn";



?>

The output was very surprising:
Memory used: 41,772 bytes

type: array

count: 420



Memory used: 4,951,248 bytes

Um, whoa. 5MB of memory just for including this file. The file itself is just 326k. Needless to say, that is bad. We include that file quite liberally. I decided to see if other methods of storing that would be better. First I tried var_export format.
Memory used: 41,784 bytes

type: array

count: 420



Memory used: 1,212,076 bytes

Well, that is much better. But, it took some fiddling to get it right. var_export does not export reference notation. The children arrays were being fully expanded and not made references. Without the references, the code was using 8MB of memory. That was much worse. Also, this is not really all that readable like the raw code version. If we can't read it, it may as well be serialized. So, I tried it serialized.
Memory used: 41,764 bytes

type: array

count: 420



Memory used: 907,668 bytes

That is by far the best result. FWIW, timing the code showed that the var_export format was fastest and serializing was slowest. However, it was just .04 seconds faster (.047 vs. .089) including the PHP start up time. I will take that for the memory savings and ease of creation.

I am going to pose a question about this on the internals list and see if this is expected behavior or if it is a shocker to them as well.