Description:
------------
The performance of serialize() decreases if the data is deeply nested, eg. consists of array inside an array.
Surprisingly, json_encode() is not affected by this issue, thus I'd say there should be some optimizations not used in serialize()

serialize() performs great on arrays structured (key => val), but fails ion structs like

array(
    array(key => val)
    ...
)

Test script:
---------------
Benchmark json_encode() vs. serialize() on the following data:

$arr = array();
for($i = 0; $i < $num_size; $i++) {
	$arr[md5($i)] = array(
		sha1($i),
		SplFixedArray::fromArray(
			array_fill(
				0, 10, hash('sha256', rand(0, $num_size))
			)
		)
	);
}


Expected result:
----------------
More or less the same same time comsumed.

Actual result:
--------------
json_encode() is >3 times faster than serialize().
This increases if the arrays nesting level increases.