groupByAuthor

<?php

function groupByOwners(array $files) : array {
    $data = [];
    foreach($files as $key => $val) {
      $data[$val][] = $key;
    }
    
    return $data;
}

$files = array
(
    "Input.txt" => "Randy",
    "Code.py" => "Stan",
    "Output.txt" => "Randy"
);
var_dump(groupByOwners($files));