php array_multisort(): Array sizes are inconsistent

I think you're taking it the wrong way. array_multisort is not what would be a "sort by" in other languages (i.e: sort array elements by some properties), instead it sorts the first array, and reverberate that order to all following arrays. And in case of equality it checks the corresponding values of the second arrays, etc...

If you want to order your example by score (desc), then by game played, then by index (and then by name, but this should never happen since indexes are uniques) you should do:

$array= Array (
    "Arnold" => Array ( "index" => 2, "games_played" => 1, "score" => 5 ),
    "Chris" => Array ( "index" => 1, "games_played" => 1, "score" => 5 ),
    "Mike" => Array ( "index" => 0, "games_played" => 2, "score" => 5 )
);
$names = [];
$indexes = [];
$games_played = [];
$scores = [];
foreach ($array as $name => $player) {
    $names[] = $name;
    $indexes[] = $player['index'];
    $games_played[] = $player['games_played'];
    $scores[] = $player['score'];
}
array_multisort(
    $scores, SORT_DESC,
    $games_played,
    $indexes,
    $names,
    $array /* This line will sort the initial array as well */
);