data_convert.php

<?php
$arr = array(
	array("id"=>1,'num'=>5,'period'=>3,'price'=>5),
	array("id"=>2,'num'=>10,'period'=>3,'price'=>5),
	array("id"=>3,'num'=>15,'period'=>9,'price'=>20)
);

$tmpData = array();
foreach($arr as $item) {
	if(isset($tmpData[$item['period']])) {
		if(isset($tmpData[$item['period']][$item['price']])) {
			$tmpData[$item['period']][$item['price']] += $item['num'];
		} else {
			$tmpData[$item['period']][$item['price']] = $item['num'];
		}
	} else {
		$tmpData[$item['period']] = array($item['price'] => $item['num']);
	}
}

$outputData = array();
foreach($tmpData as $period => $itemArr) {
	foreach($itemArr as $price => $num) {
		$data = array();
		$data['num'] = $num;
		$data['period'] = $period;
		$data['price'] = $price;
		
		$outputData[] = $data;
	}
}

print_r($outputData);