data_convert.php

<?php

$old = [
["id"=>1,'num'=>5,'period'=>35,'price'=>55],
["id"=>2,'num'=>10,'period'=>355,'price'=>5],
["id"=>3,'num'=>15,'period'=>9,'price'=>20]
];

$new = [];
foreach($old as $k => $v){
if(isset($new[$v['period']])){
$new[$v['period'] . $v['price']]['num'] += $v['num'];
}else{
unset($v['id']);
$new[$v['period'] . $v['price']] = $v;
}
}

var_dump($new);