<?php

require_once('./json.php'); //Importing the data file assuming it lies in the same directory as challenge.php file

$peopleArray = json_decode($people)->data;

//Making an array with all the emails and adding an extra property to $peopleArray
foreach($peopleArray as $key => $peopleObj){
    $emails[] = $peopleObj->email;
    $peopleObj->name = $peopleObj->first_name.' '.$peopleObj->last_name;
}

//Sorting $peopleArray by age in descending order.
usort($peopleArray, function($prev,$next){
    if ($prev->age === $next->age) {
        return 0;
    }
    return ($prev->age < $next->age) ?  1 : -1;
});

$emails_list = implode($emails,','); //Converting array to comma saperated elements.
$newData = json_encode(['data' => $peopleArray]); //Creating json using the mutated $peopleArray.