Structuring OOP Php methods and objects when using a JSON API call - php

Question

I am making an application that pulls data through JSON with CURL. I'm using PHP and my script currently creates 2 different objects to contain the curl requests. I've got the data in a readable format, but I need to divide an average from the first call by an average from the second call.
This average is of equal relevance to each of the calls - and I'm new to working with objects and classes so am struggling to understand how to structure this in the right way. The following line of code doesn't work - but, if you read on it will hopefully help to explain what I'm trying to do.
$yield = object1->calculate_average($list)->average['price']/object2->calculate_average($list)->average['price'];
I want to code the $yield variable in to the listing class as a further method that will display when I call $anobject->yield. Should I access this via a) object1, b) object2, or c) it's own object (which seems excessive..as it will only be used occasionally). Is there a right way to do this, or does it not really matter?
My query is general (what is the best approach to using objects when an object appears to be as relevant to all options, and is it possible to reference arrays like this: calculate_average($list)->average['price']) as much as it is specific (how can I fix this script)
When this has come up before I've tended to just regrab the data from the source and fudge it, but because it's a JSON call and therefore takes a bit longer I've been prompted to find a better solution...
A simplified version of my best effort so far is below.
I've assumed that I need to use foreach on the json outputs:
class listing {
//This takes a stdClass object from the JSON and prepares it to be served
public function output_list($data) {
$price = "0";
$vals = "0";
$list = array();
foreach ($data->listing as $item) {
//Calculate average price
$price = $price + $item->price;
$vals = $vals + 1;
//Display information about each item
$list[$vals][] = "Price: £" . $item->price . " and various data";
$list[$vals][] = ...;
}
return $list;
}
// This function calculates equations on the data which I read above
public function calculate_average($list) {
$average = array();
$total_price = array_column($list, '1'); //Find the total sum of list[?][1] which = price
$vals = count($list); //count the number of values in the list
$average['denominator'] = $vals;
$average['week'] = $total_price / $average['denominator'];
//And a number of other calculations within the array...
return $average;
}
}
I then want to call these by using the following code:
$object1 = new listing();
$list1 = $object1->output_list($response); ////Output formatted data
print_r($list1);
$average = $object1->calculate_average($list);
$object2 = new listing();
$list2 = $object2->output_list($response); ////Output formatted data
print_r($list2);
$average = $object2->calculate_average($list);
The other thing is that $total_price = array_column($list,'1') needs me to have php 5.5 - which I don't have, so I need to find an alternative - and that makes me question whether my array approach is the right one in the first place. [I need to sum every [?][1] reference in the multi-dimensional array I've made].
If you're able to help I'd be so grateful - I've been trying to get my head around objects and classes for weeks!