Missing values from the same key after using array_flip function

I'm trying to flip array but it missed value from the key with the same name. What do I have to use, to add couple values to key which occur multiple times in array?

For example, for

[
    "Input.txt" => "Randy",
    "Code.py" => "Stan",
    "Output.txt" => "Randy"
]

the groupByOwners function should return

[
    "Randy" => ["Input.txt", "Output.txt"],
    "Stan" => ["Code.py"]
]

The current code:

class FileOwners
{
    static $files;
    public static function groupByOwners($files)
    {
       $flip = array_flip($files);
        print_r($flip);
    }
}

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

My function return Array ( [Randy] => Output.txt [Stan] => Code.py ) NULL.

So value "Input.txt" is missing. It has to be the same key for both values, so how can I put "Input.txt" with "Output.txt" in array for key [Randy]?

You will have to loop it yourself and build a new array:

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

$new_files = array();

foreach($files as $k=>$v)
{
    $new_files[$v][] = $k;
}

print_r($new_files);